scryer-prolog/src/machine/loader.rs

use crate::arena::*;
use crate::atom_table::*;
use crate::forms::*;
use crate::heap_iter::*;
use crate::indexing::*;
use crate::instructions::*;
use crate::machine::load_state::*;
use crate::machine::machine_errors::*;
use crate::machine::machine_indices::*;
use crate::machine::preprocessor::*;
use crate::machine::term_stream::*;
use crate::machine::*;
use crate::parser::ast::*;
use crate::types::*;

use indexmap::IndexSet;

use std::cell::Cell;
use std::collections::VecDeque;
use std::convert::TryFrom;
use std::fmt;
use std::mem;
use std::ops::{Deref, DerefMut};

/*
 * The loader compiles Prolog terms read from a TermStream instance,
 * which may be incremental or monolithic. The monolithic term stream
 * reads from a file. It's used only to bootstrap Scryer at
 * start-up. Once Scryer is bootstrapped, all compilation and loading
 * work is divided between loader.pl and loader.rs.
 *
 * loader.pl does a few high-level things more easily handled from
 * Prolog that are not supported (or needed) during bootstrapping:
 * term and goal expansion, loading modules from different streams,
 * and verifying certain kinds of declarations.
 *
 * Since the loader can operate incrementally, it uses an intermittent
 * structure to rebuild the loader between invocations. Preprocessor
 * needs access to a &'a mut Machine for as long as it lives, and we
 * can't have copies of &'a mut Machine distributed among multiple
 * owners.
 *
 * When loading a module, we modify the records of the WAM with the
 * location of new predicates, with new meta-predicate information,
 * new term and goal expansions, new dynamic clauses, etc. Should the
 * loader fail later, all changes must be rolled back, restoring the
 * WAM to its prior state. Retraction records describe individual changes
 * made by the loader, and they may be used later.
 */

#[derive(Debug)]
pub(crate) enum RetractionRecord {
    AddedMetaPredicate(Atom, PredicateKey),
    ReplacedMetaPredicate(Atom, Atom, Vec<MetaSpec>),
    AddedModule(Atom),
    ReplacedModule(ModuleDecl, ListingSource, LocalExtensiblePredicates),
    AddedModuleOp(Atom, OpDecl),
    ReplacedModuleOp(Atom, OpDecl, OpDesc),
    AddedModulePredicate(Atom, PredicateKey),
    ReplacedModulePredicate(Atom, PredicateKey, IndexPtr),
    AddedDiscontiguousPredicate(CompilationTarget, PredicateKey),
    AddedDynamicPredicate(CompilationTarget, PredicateKey),
    AddedMultifilePredicate(CompilationTarget, PredicateKey),
    AddedUserOp(OpDecl),
    ReplacedUserOp(OpDecl, OpDesc),
    AddedExtensiblePredicate(CompilationTarget, PredicateKey),
    AddedUserPredicate(PredicateKey),
    ReplacedUserPredicate(PredicateKey, IndexPtr),
    AddedIndex(OptArgIndexKey, usize), //, Vec<usize>),
    RemovedIndex(usize, OptArgIndexKey, usize),
    ReplacedChoiceOffset(usize, usize),
    AppendedTrustMe(usize, usize, bool),
    ReplacedSwitchOnTermVarIndex(usize, IndexingCodePtr),
    ModifiedTryMeElse(usize, usize),
    ModifiedRetryMeElse(usize, usize),
    ModifiedRevJmpBy(usize, usize),
    SkeletonClausePopBack(CompilationTarget, PredicateKey),
    SkeletonClausePopFront(CompilationTarget, PredicateKey),
    SkeletonLocalClauseClausePopBack(CompilationTarget, CompilationTarget, PredicateKey),
    SkeletonLocalClauseClausePopFront(CompilationTarget, CompilationTarget, PredicateKey),
    SkeletonLocalClauseTruncateBack(CompilationTarget, CompilationTarget, PredicateKey, usize),
    SkeletonClauseTruncateBack(CompilationTarget, PredicateKey, usize),
    SkeletonClauseStartReplaced(CompilationTarget, PredicateKey, usize, usize),
    RemovedDynamicSkeletonClause(CompilationTarget, PredicateKey, usize, usize),
    RemovedSkeletonClause(
        CompilationTarget,
        PredicateKey,
        usize,
        ClauseIndexInfo,
        usize,
    ),
    ReplacedIndexingLine(usize, Vec<IndexingLine>),
    RemovedLocalSkeletonClauseLocations(
        CompilationTarget,
        CompilationTarget,
        PredicateKey,
        VecDeque<usize>,
    ),
    RemovedSkeleton(CompilationTarget, PredicateKey, PredicateSkeleton),
    ReplacedDynamicElseOffset(usize, usize),
    AppendedNextOrFail(usize, NextOrFail),
}

/*
 * Retractions to be performed on rollback are represented by
 * individual records, and the original extent of the code vector of
 * the IndexStore, of which there are several (one per module).  The
 * "extent" of a code vector is its length prior to an attempted
 * module load. The only code vector of the WAM's IndexStore, "code",
 * is shared by all modules, including the default "user" module.
 */

pub(super) struct RetractionInfo {
    orig_code_extent: usize,
    records: Vec<RetractionRecord>,
}

impl RetractionInfo {
    #[inline]
    pub(super) fn new(orig_code_extent: usize) -> Self {
        Self {
            orig_code_extent,
            records: vec![],
        }
    }

    #[inline]
    pub(crate) fn push_record(&mut self, record: RetractionRecord) {
        self.records.push(record);
    }

    #[inline]
    pub(crate) fn reset(&mut self, code_len: usize) -> Self {
        let orig_code_extent = self.orig_code_extent;
        self.orig_code_extent = code_len;

        Self {
            orig_code_extent,
            records: mem::replace(&mut self.records, vec![]),
        }
    }
}

impl<'a, LS: LoadState<'a>> Drop for Loader<'a, LS> {
    fn drop(&mut self) {
        if LS::should_drop_load_state(self) {
            LS::reset_machine(self);
        }
    }
}

#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq)]
pub enum CompilationTarget {
    Module(Atom),
    User,
}

impl fmt::Display for CompilationTarget {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            CompilationTarget::User => write!(f, "user"),
            CompilationTarget::Module(ref module_name) => write!(f, "{}", module_name.as_str()),
        }
    }
}

impl Default for CompilationTarget {
    #[inline]
    fn default() -> Self {
        CompilationTarget::User
    }
}

impl CompilationTarget {
    #[inline]
    pub(crate) fn module_name(&self) -> Atom {
        match self {
            CompilationTarget::User => {
                atom!("user")
            }
            CompilationTarget::Module(module_name) => *module_name,
        }
    }
}

pub struct PredicateQueue {
    pub(super) predicates: Vec<Term>,
    pub(super) compilation_target: CompilationTarget,
}

impl PredicateQueue {
    #[inline]
    pub(super) fn push(&mut self, clause: Term) {
        self.predicates.push(clause);
    }

    #[inline]
    pub(crate) fn first(&self) -> Option<&Term> {
        self.predicates.first()
    }

    #[inline]
    pub(crate) fn is_empty(&self) -> bool {
        self.predicates.is_empty()
    }

    #[inline]
    pub(super) fn take(&mut self) -> Self {
        Self {
            predicates: mem::replace(&mut self.predicates, vec![]),
            compilation_target: self.compilation_target.clone(),
        }
    }

    #[inline]
    pub(super) fn len(&self) -> usize {
        self.predicates.len()
    }
}

#[macro_export]
macro_rules! predicate_queue {
    [$($v:expr),*] => (
        PredicateQueue {
            predicates: vec![$($v,)*],
            compilation_target: CompilationTarget::default(),
        }
    )
}

pub type LiveLoadState = LoadStatePayload<LiveTermStream>;

pub struct BootstrappingLoadState<'a>(
    pub LoadStatePayload<BootstrappingTermStream<'a>>
);

impl<'a> Deref for BootstrappingLoadState<'a> {
    type Target = LoadStatePayload<BootstrappingTermStream<'a>>;

    #[inline(always)]
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<'a> DerefMut for BootstrappingLoadState<'a> {
    #[inline(always)]
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}

pub trait LoadState<'a>: Sized {
    type Evacuable;
    type TS: TermStream;
    type LoaderFieldType: DerefMut<Target=LoadStatePayload<Self::TS>>;

    fn new(machine_st: &'a mut MachineState, payload: LoadStatePayload<Self::TS>) -> Self::LoaderFieldType;
    fn evacuate(loader: Loader<'a, Self>) -> Result<Self::Evacuable, SessionError>;
    fn should_drop_load_state(loader: &Loader<'a, Self>) -> bool;
    fn reset_machine(loader: &mut Loader<'a, Self>);
    fn machine_st(loader: &mut Self::LoaderFieldType) -> &mut MachineState;
    fn err_on_builtin_overwrite(
        loader: &Loader<'a, Self>,
        key: PredicateKey,
    ) -> Result<(), SessionError>;
}

pub struct LiveLoadAndMachineState<'a> {
    load_state: TypedArenaPtr<LiveLoadState>,
    machine_st: &'a mut MachineState,
}

impl<'a> Deref for LiveLoadAndMachineState<'a> {
    type Target = LiveLoadState;

    #[inline(always)]
    fn deref(&self) -> &Self::Target {
        &self.load_state
    }
}

impl<'a> DerefMut for LiveLoadAndMachineState<'a> {
    #[inline(always)]
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.load_state
    }
}

impl<'a> LoadState<'a> for LiveLoadAndMachineState<'a> {
    type TS = LiveTermStream;
    type LoaderFieldType = LiveLoadAndMachineState<'a>;
    type Evacuable = TypedArenaPtr<LiveLoadState>;

    #[inline(always)]
    fn new(machine_st: &'a mut MachineState, payload: LoadStatePayload<Self::TS>) -> Self::LoaderFieldType {
        let load_state = arena_alloc!(payload, &mut machine_st.arena);
        LiveLoadAndMachineState { load_state, machine_st }
    }

    #[inline(always)]
    fn evacuate(mut loader: Loader<'a, Self>) -> Result<Self::Evacuable, SessionError> {
        loader.payload.load_state.set_tag(ArenaHeaderTag::InactiveLoadState);
        Ok(loader.payload.load_state)
    }

    #[inline(always)]
    fn should_drop_load_state(loader: &Loader<'a, Self>) -> bool {
        loader.payload.load_state.get_tag() == ArenaHeaderTag::LiveLoadState
    }

    #[inline(always)]
    fn reset_machine(loader: &mut Loader<'a, Self>) {
        if loader.payload.load_state.get_tag() != ArenaHeaderTag::Dropped {
            loader.payload.load_state.set_tag(ArenaHeaderTag::Dropped);
            loader.reset_machine();
        }
    }

    #[inline(always)]
    fn machine_st(loader: &mut Self::LoaderFieldType) -> &mut MachineState {
        loader.machine_st
    }

    #[inline(always)]
    fn err_on_builtin_overwrite(
        loader: &Loader<'a, Self>,
        key: PredicateKey,
    ) -> Result<(), SessionError> {
        if ClauseType::is_inbuilt(key.0, key.1) {
            return Err(SessionError::CannotOverwriteBuiltIn(key));
        }

        if let Some(builtins) = loader.wam_prelude.indices.modules.get(&atom!("builtins")) {
            if builtins.module_decl.exports.contains(&ModuleExport::PredicateKey(key)) {
                return Err(SessionError::CannotOverwriteBuiltIn(key));
            }
        }

        Ok(())
    }
}

impl<'a> LoadState<'a> for BootstrappingLoadState<'a> {
    type TS = BootstrappingTermStream<'a>;
    type LoaderFieldType = BootstrappingLoadState<'a>;
    type Evacuable = CompilationTarget;

    #[inline(always)]
    fn new(_: &'a mut MachineState, payload: LoadStatePayload<Self::TS>) -> Self::LoaderFieldType {
        BootstrappingLoadState(payload)
    }

    fn evacuate(mut loader: Loader<'a, Self>) -> Result<Self::Evacuable, SessionError> {
        if !loader.payload.predicates.is_empty() {
            loader.compile_and_submit()?;
        }

        let repo_len = loader.wam_prelude.code.len();

        loader
            .payload
            .retraction_info
            .reset(repo_len);

        loader.remove_module_op_exports();

        Ok(loader.payload.compilation_target)
    }

    #[inline(always)]
    fn should_drop_load_state(_loader: &Loader<'a, Self>) -> bool {
        true
    }

    #[inline(always)]
    fn reset_machine(loader: &mut Loader<'a, Self>) {
        loader.reset_machine();
    }

    #[inline(always)]
    fn machine_st(loader: &mut Self::LoaderFieldType) -> &mut MachineState {
        &mut loader.term_stream.parser.lexer.machine_st
    }

    #[inline(always)]
    fn err_on_builtin_overwrite(
        _loader: &Loader<'a, Self>,
        _key: PredicateKey,
    ) -> Result<(), SessionError> {
        Ok(())
    }
}

pub struct InlineLoadState<'a> {
    machine_st: &'a mut MachineState,
    pub payload: LoadStatePayload<InlineTermStream>,
}

impl<'a> Deref for InlineLoadState<'a> {
    type Target = LoadStatePayload<InlineTermStream>;

    #[inline(always)]
    fn deref(&self) -> &Self::Target {
        &self.payload
    }
}

impl<'a> DerefMut for InlineLoadState<'a> {
    #[inline(always)]
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.payload
    }
}

impl<'a> LoadState<'a> for InlineLoadState<'a> {
    type TS = InlineTermStream;
    type LoaderFieldType = InlineLoadState<'a>;
    type Evacuable = ();

    #[inline(always)]
    fn new(machine_st: &'a mut MachineState, payload: LoadStatePayload<Self::TS>) -> Self::LoaderFieldType {
	InlineLoadState { machine_st, payload }
    }

    fn evacuate(_loader: Loader<'a, Self>) -> Result<Self::Evacuable, SessionError> {
	Ok(())
    }

    #[inline(always)]
    fn should_drop_load_state(_loader: &Loader<'a, Self>) -> bool {
	false
    }

    #[inline(always)]
    fn reset_machine(_loader: &mut Loader<'a, Self>) {
    }

    #[inline(always)]
    fn machine_st(load_state: &mut Self::LoaderFieldType) -> &mut MachineState {
        &mut load_state.machine_st
    }

    #[inline(always)]
    fn err_on_builtin_overwrite(
        _loader: &Loader<'a, Self>,
        _key: PredicateKey,
    ) -> Result<(), SessionError> {
        Ok(())
    }
}

pub struct Loader<'a, LS: LoadState<'a>> {
    pub(super) payload: LS::LoaderFieldType,
    pub(super) wam_prelude: MachinePreludeView<'a>,
}

impl<'a, LS: LoadState<'a>> Loader<'a, LS> {
    #[inline]
    pub(super) fn new(wam: &'a mut Machine, term_stream: <LS as LoadState<'a>>::TS) -> Self {
        let payload = LoadStatePayload::new(wam.code.len(), term_stream);
        let (wam_prelude, machine_st) = wam.prelude_view_and_machine_st();

        Self {
            payload: LS::new(machine_st, payload),
            wam_prelude,
        }
    }

    pub(crate) fn read_term_from_heap(&mut self, r: RegType) -> Result<Term, SessionError> {
        let machine_st = LS::machine_st(&mut self.payload);
        let cell = machine_st[r];

        machine_st.read_term_from_heap(cell)
    }

    pub(crate) fn load(mut self) -> Result<LS::Evacuable, SessionError> {
        while let Some(decl) = self.dequeue_terms()? {
            self.load_decl(decl)?;
        }

        LS::evacuate(self)
    }

    fn dequeue_terms(&mut self) -> Result<Option<Declaration>, SessionError> {
        while !self.payload.term_stream.eof()? {
            let load_state = &mut self.payload;
            let compilation_target = &load_state.compilation_target;
            let composite_op_dir = self.wam_prelude.composite_op_dir(compilation_target);

            let term = load_state.term_stream.next(&composite_op_dir)?;

            if !term.is_consistent(&load_state.predicates) {
                self.compile_and_submit()?;
            }

            let term = match term {
                Term::Clause(_, name, terms) if name == atom!(":-") && terms.len() == 1 => {
                    return Ok(Some(setup_declaration(self, terms)?));
                }
                term => term,
            };

            self.payload.predicates.push(term);
        }

        Ok(None)
    }

    pub(super) fn load_decl(&mut self, decl: Declaration) -> Result<(), SessionError> {
        match decl {
            Declaration::Dynamic(name, arity) => {
                let compilation_target = self.payload.compilation_target;
                self.add_dynamic_predicate(compilation_target, name, arity)?;
            }
            Declaration::MetaPredicate(module_name, name, meta_specs) => {
                self.add_meta_predicate_record(module_name, name, meta_specs);
            }
            Declaration::Module(module_decl) => {
                self.payload.compilation_target = CompilationTarget::Module(module_decl.name);
                self.payload.predicates.compilation_target = self.payload.compilation_target;

                let listing_src = self.payload.term_stream.listing_src().clone();
                self.add_module(module_decl, listing_src);
            }
            Declaration::NonCountedBacktracking(name, arity) => {
                self.payload.non_counted_bt_preds.insert((name, arity));
            }
            Declaration::Op(op_decl) => {
                self.add_op_decl(&op_decl);
            }
            Declaration::UseModule(module_src) => {
                self.use_module(module_src)?;
            }
            Declaration::UseQualifiedModule(module_src, exports) => {
                self.use_qualified_module(module_src, exports)?;
            }
        }

        Ok(())
    }

    fn reset_machine(&mut self) {
        while let Some(record) = self.payload.retraction_info.records.pop() {
            match record {
                RetractionRecord::AddedMetaPredicate(target_module_name, key) => {
                    match target_module_name {
                        atom!("user") => {
                            self.wam_prelude.indices.meta_predicates.remove(&key);
                        }
                        _ => match self.wam_prelude.indices.modules.get_mut(&target_module_name) {
                            Some(ref mut module) => {
                                module.meta_predicates.remove(&key);
                            }
                            _ => {
                                unreachable!()
                            }
                        },
                    }
                }
                RetractionRecord::ReplacedMetaPredicate(target_module_name, name, meta_specs) => {
                    match target_module_name {
                        atom!("user") => {
                            self.wam_prelude
                                .indices
                                .meta_predicates
                                .insert((name, meta_specs.len()), meta_specs);
                        }
                        _ => match self.wam_prelude.indices.modules.get_mut(&target_module_name) {
                            Some(ref mut module) => {
                                module
                                    .meta_predicates
                                    .insert((name, meta_specs.len()), meta_specs);
                            }
                            _ => {
                                unreachable!()
                            }
                        },
                    }
                }
                RetractionRecord::AddedModule(module_name) => {
                    self.wam_prelude.indices.modules.remove(&module_name);
                }
                RetractionRecord::ReplacedModule(
                    module_decl,
                    listing_src,
                    local_extensible_predicates,
                ) => match self.wam_prelude.indices.modules.get_mut(&module_decl.name) {
                    Some(ref mut module) => {
                        module.module_decl = module_decl;
                        module.listing_src = listing_src;
                        module.local_extensible_predicates = local_extensible_predicates;
                    }
                    _ => {
                        unreachable!()
                    }
                },
                RetractionRecord::AddedDiscontiguousPredicate(compilation_target, key) => {
                    match compilation_target {
                        CompilationTarget::User => {
                            self.wam_prelude
                                .indices
                                .extensible_predicates
                                .get_mut(&key)
                                .map(|skeleton| {
                                    skeleton.core.is_discontiguous = false;
                                });
                        }
                        CompilationTarget::Module(module_name) => {
                            match self.wam_prelude.indices.modules.get_mut(&module_name) {
                                Some(ref mut module) => {
                                    module.extensible_predicates.get_mut(&key).map(|skeleton| {
                                        skeleton.core.is_discontiguous = false;
                                    });
                                }
                                None => {}
                            }
                        }
                    }
                }
                RetractionRecord::AddedDynamicPredicate(compilation_target, key) => {
                    match compilation_target {
                        CompilationTarget::User => {
                            self.wam_prelude
                                .indices
                                .extensible_predicates
                                .get_mut(&key)
                                .map(|skeleton| {
                                    skeleton.core.is_dynamic = false;
                                });
                        }
                        CompilationTarget::Module(module_name) => {
                            match self.wam_prelude.indices.modules.get_mut(&module_name) {
                                Some(ref mut module) => {
                                    module.extensible_predicates.get_mut(&key).map(|skeleton| {
                                        skeleton.core.is_dynamic = false;
                                        skeleton.core.retracted_dynamic_clauses = None;
                                    });
                                }
                                None => {}
                            }
                        }
                    }
                }
                RetractionRecord::AddedMultifilePredicate(compilation_target, key) => {
                    match compilation_target {
                        CompilationTarget::User => {
                            self.wam_prelude
                                .indices
                                .extensible_predicates
                                .get_mut(&key)
                                .map(|skeleton| {
                                    skeleton.core.is_multifile = false;
                                });
                        }
                        CompilationTarget::Module(module_name) => {
                            match self.wam_prelude.indices.modules.get_mut(&module_name) {
                                Some(ref mut module) => {
                                    module.extensible_predicates.get_mut(&key).map(|skeleton| {
                                        skeleton.core.is_multifile = false;
                                    });
                                }
                                None => {}
                            }
                        }
                    }
                }
                RetractionRecord::AddedModuleOp(module_name, mut op_decl) => {
                    match self.wam_prelude.indices.modules.get_mut(&module_name) {
                        Some(ref mut module) => {
                            op_decl.remove(&mut module.op_dir);
                        }
                        None => {}
                    }
                }
                RetractionRecord::ReplacedModuleOp(module_name, mut op_decl, op_desc) => {
                    match self.wam_prelude.indices.modules.get_mut(&module_name) {
                        Some(ref mut module) => {
                            op_decl.op_desc = op_desc;
                            op_decl.insert_into_op_dir(&mut module.op_dir);
                        }
                        None => {}
                    }
                }
                RetractionRecord::AddedModulePredicate(module_name, key) => {
                    match self.wam_prelude.indices.modules.get_mut(&module_name) {
                        Some(ref mut module) => {
                            module.code_dir.remove(&key);
                        }
                        None => {}
                    }
                }
                RetractionRecord::ReplacedModulePredicate(module_name, key, old_code_idx) => {
                    match self.wam_prelude.indices.modules.get_mut(&module_name) {
                        Some(ref mut module) => {
                            module
                                .code_dir
                                .get_mut(&key)
                                .map(|code_idx| code_idx.set(old_code_idx));
                        }
                        None => {}
                    }
                }
                RetractionRecord::AddedExtensiblePredicate(compilation_target, key) => {
                    self.wam_prelude
                        .indices
                        .remove_predicate_skeleton(&compilation_target, &key);
                }
                RetractionRecord::AddedUserOp(mut op_decl) => {
                    op_decl.remove(&mut self.wam_prelude.indices.op_dir);
                }
                RetractionRecord::ReplacedUserOp(mut op_decl, op_desc) => {
                    op_decl.op_desc = op_desc;
                    op_decl.insert_into_op_dir(&mut self.wam_prelude.indices.op_dir);
                }
                RetractionRecord::AddedUserPredicate(key) => {
                    self.wam_prelude.indices.code_dir.remove(&key);
                }
                RetractionRecord::ReplacedUserPredicate(key, old_code_idx) => {
                    self.wam_prelude
                        .indices
                        .code_dir
                        .get_mut(&key)
                        .map(|code_idx| code_idx.set(old_code_idx));
                }
                RetractionRecord::AddedIndex(index_key, clause_loc) => {
                    if let Some(index_loc) = index_key.switch_on_term_loc() {
                        let indexing_code = match &mut self.wam_prelude.code[index_loc] {
                            Instruction::IndexingCode(indexing_code) => indexing_code,
                            _ => {
                                unreachable!()
                            }
                        };

                        match index_key {
                            OptArgIndexKey::Literal(
                                _,
                                index_loc,
                                constant,
                                overlapping_constants,
                            ) => {
                                remove_constant_indices(
                                    constant,
                                    &overlapping_constants,
                                    indexing_code,
                                    clause_loc - index_loc, // WAS: &inner_index_locs,
                                );
                            }
                            OptArgIndexKey::Structure(_, index_loc, name, arity) => {
                                remove_structure_index(
                                    name,
                                    arity,
                                    indexing_code,
                                    clause_loc - index_loc, // WAS: &inner_index_locs,
                                );
                            }
                            OptArgIndexKey::List(_, index_loc) => {
                                remove_list_index(
                                    indexing_code,
                                    clause_loc - index_loc, // WAS: &inner_index_locs,
                                );
                            }
                            OptArgIndexKey::None => {
                                unreachable!();
                            }
                        }
                    }
                }
                RetractionRecord::RemovedIndex(_index_loc, _index_key, _clause_loc) => {
                    // TODO: this needs to be fixed! RemovedIndex doesn't provide
                    // enough information to restore the index. Correct that, then
                    // write the retraction logic of this arm.
                }
                RetractionRecord::ReplacedChoiceOffset(instr_loc, offset) => {
                    match self.wam_prelude.code[instr_loc] {
                        Instruction::TryMeElse(ref mut o) |
                        Instruction::RetryMeElse(ref mut o) |
                        Instruction::DefaultRetryMeElse(ref mut o) => {
                            *o = offset;
                        }
                        _ => {
                            unreachable!();
                        }
                    }
                }
                RetractionRecord::AppendedTrustMe(instr_loc, offset, is_default) => {
                    self.wam_prelude.code[instr_loc] = if is_default {
                        Instruction::DefaultTrustMe(offset)
                    } else {
                        Instruction::TrustMe(offset)
                    };
                }
                RetractionRecord::ReplacedSwitchOnTermVarIndex(index_loc, old_v) => {
                    match self.wam_prelude.code[index_loc] {
                        Instruction::IndexingCode(ref mut indexing_code) => match &mut indexing_code[0] {
                            IndexingLine::Indexing(IndexingInstruction::SwitchOnTerm(
                                _,
                                ref mut v,
                                ..,
                            )) => {
                                *v = old_v;
                            }
                            _ => {}
                        },
                        _ => {}
                    }
                }
                RetractionRecord::ModifiedTryMeElse(instr_loc, o) => {
                    self.wam_prelude.code[instr_loc] = Instruction::TryMeElse(o);
                }
                RetractionRecord::ModifiedRetryMeElse(instr_loc, o) => {
                    self.wam_prelude.code[instr_loc] = Instruction::RetryMeElse(o);
                }
                RetractionRecord::ModifiedRevJmpBy(instr_loc, o) => {
                    self.wam_prelude.code[instr_loc] = Instruction::RevJmpBy(o);
                }
                RetractionRecord::SkeletonClausePopBack(compilation_target, key) => {
                    match self
                        .wam_prelude
                        .indices
                        .get_predicate_skeleton_mut(&compilation_target, &key)
                    {
                        Some(skeleton) => {
                            skeleton.clauses.pop_back();
                            skeleton.core.clause_clause_locs.pop_back();
                        }
                        None => {}
                    }
                }
                RetractionRecord::SkeletonClausePopFront(compilation_target, key) => {
                    match self
                        .wam_prelude
                        .indices
                        .get_predicate_skeleton_mut(&compilation_target, &key)
                    {
                        Some(skeleton) => {
                            skeleton.clauses.pop_front();
                            skeleton.core.clause_clause_locs.pop_front();
                            skeleton.core.clause_assert_margin -= 1;
                        }
                        None => {}
                    }
                }
                RetractionRecord::SkeletonLocalClauseClausePopFront(
                    src_compilation_target,
                    local_compilation_target,
                    key,
                ) => {
                    let listing_src_file_name = self.listing_src_file_name();

                    match self.wam_prelude.indices.get_local_predicate_skeleton_mut(
                        src_compilation_target,
                        local_compilation_target,
                        listing_src_file_name,
                        key,
                    ) {
                        Some(skeleton) => {
                            skeleton.clause_clause_locs.pop_front();
                        }
                        None => {}
                    }
                }
                RetractionRecord::SkeletonLocalClauseClausePopBack(
                    src_compilation_target,
                    local_compilation_target,
                    key,
                ) => {
                    let listing_src_file_name = self.listing_src_file_name();

                    match self.wam_prelude.indices.get_local_predicate_skeleton_mut(
                        src_compilation_target,
                        local_compilation_target,
                        listing_src_file_name,
                        key,
                    ) {
                        Some(skeleton) => {
                            skeleton.clause_clause_locs.pop_back();
                        }
                        None => {}
                    }
                }
                RetractionRecord::SkeletonLocalClauseTruncateBack(
                    src_compilation_target,
                    local_compilation_target,
                    key,
                    len,
                ) => {
                    let listing_src_file_name = self.listing_src_file_name();

                    match self.wam_prelude.indices.get_local_predicate_skeleton_mut(
                        src_compilation_target,
                        local_compilation_target,
                        listing_src_file_name,
                        key,
                    ) {
                        Some(skeleton) => {
                            skeleton.clause_clause_locs.truncate(len);
                        }
                        None => {}
                    }
                }
                RetractionRecord::SkeletonClauseTruncateBack(compilation_target, key, len) => {
                    match self
                        .wam_prelude
                        .indices
                        .get_predicate_skeleton_mut(&compilation_target, &key)
                    {
                        Some(skeleton) => {
                            skeleton.clauses.truncate(len);
                            skeleton.core.clause_clause_locs.truncate(len);
                        }
                        None => {}
                    }
                }
                RetractionRecord::SkeletonClauseStartReplaced(
                    compilation_target,
                    key,
                    target_pos,
                    clause_start,
                ) => {
                    match self
                        .wam_prelude
                        .indices
                        .get_predicate_skeleton_mut(&compilation_target, &key)
                    {
                        Some(skeleton) => {
                            skeleton.clauses[target_pos].clause_start = clause_start;
                        }
                        None => {}
                    }
                }
                RetractionRecord::RemovedDynamicSkeletonClause(
                    compilation_target,
                    key,
                    target_pos,
                    clause_clause_loc,
                ) => {
                    match self
                        .wam_prelude
                        .indices
                        .get_predicate_skeleton_mut(&compilation_target, &key)
                    {
                        Some(skeleton) => {
                            if let Some(removed_clauses) = &mut skeleton.core.retracted_dynamic_clauses {
                                let clause_index_info = removed_clauses.pop().unwrap();

                                skeleton
                                    .core
                                    .clause_clause_locs
                                    .insert(target_pos, clause_clause_loc);

                                skeleton.clauses.insert(target_pos, clause_index_info);
                            }
                        }
                        None => {}
                    }
                }
                RetractionRecord::RemovedSkeletonClause(
                    compilation_target,
                    key,
                    target_pos,
                    clause_index_info,
                    clause_clause_loc,
                ) => {
                    match self
                        .wam_prelude
                        .indices
                        .get_predicate_skeleton_mut(&compilation_target, &key)
                    {
                        Some(skeleton) => {
                            skeleton
                                .core
                                .clause_clause_locs
                                .insert(target_pos, clause_clause_loc);
                            skeleton.clauses.insert(target_pos, clause_index_info);
                        }
                        None => {}
                    }
                }
                RetractionRecord::ReplacedIndexingLine(index_loc, indexing_code) => {
                    self.wam_prelude.code[index_loc] = Instruction::IndexingCode(indexing_code);
                }
                RetractionRecord::RemovedLocalSkeletonClauseLocations(
                    compilation_target,
                    local_compilation_target,
                    key,
                    clause_locs,
                ) => {
                    let listing_src_file_name = self.listing_src_file_name();

                    match self.wam_prelude.indices.get_local_predicate_skeleton_mut(
                        compilation_target,
                        local_compilation_target,
                        listing_src_file_name,
                        key,
                    ) {
                        Some(skeleton) => skeleton.clause_clause_locs = clause_locs,
                        None => {}
                    }
                }
                RetractionRecord::RemovedSkeleton(compilation_target, key, skeleton) => {
                    match compilation_target {
                        CompilationTarget::User => {
                            self.wam_prelude.indices.extensible_predicates.insert(key, skeleton);
                        }
                        CompilationTarget::Module(module_name) => {
                            if let Some(module) = self.wam_prelude.indices.modules.get_mut(&module_name) {
                                module.extensible_predicates.insert(key, skeleton);
                            }
                        }
                    }
                }
                RetractionRecord::ReplacedDynamicElseOffset(instr_loc, next) => {
                    match self.wam_prelude.code[instr_loc] {
                        Instruction::DynamicElse(
                            _,
                            _,
                            NextOrFail::Next(ref mut o),
                        )
                        | Instruction::DynamicInternalElse(
                            _,
                            _,
                            NextOrFail::Next(ref mut o),
                        ) => {
                            *o = next;
                        }
                        _ => {}
                    }
                }
                RetractionRecord::AppendedNextOrFail(instr_loc, fail) => {
                    match self.wam_prelude.code[instr_loc] {
                        Instruction::DynamicElse(
                            _,
                            _,
                            ref mut next_or_fail,
                        )
                        | Instruction::DynamicInternalElse(
                            _,
                            _,
                            ref mut next_or_fail,
                        ) => {
                            *next_or_fail = fail;
                        }
                        _ => {}
                    }
                }
            }
        }
    }

    fn extract_module_export_list_from_heap(
        &mut self,
        r: RegType,
    ) -> Result<IndexSet<ModuleExport>, SessionError> {
        let machine_st = LS::machine_st(&mut self.payload);
        let cell = machine_st[r];

        let export_list = machine_st.read_term_from_heap(cell)?;
        let atom_tbl = &mut LS::machine_st(&mut self.payload).atom_tbl;
        let export_list = setup_module_export_list(export_list, atom_tbl)?;

        Ok(export_list.into_iter().collect())
    }

    fn add_clause_clause(&mut self, term: Term) -> Result<(), CompilationError> {
        match term {
            Term::Clause(_, atom!(":-"), mut terms) if terms.len() == 2 =>
            {
                let body = terms.pop().unwrap();
                let head = terms.pop().unwrap();

                self.payload.clause_clauses.push((head, body));
            }
            head @ Term::Literal(_, Literal::Atom(..)) | head @ Term::Clause(..) => {
                let body = Term::Literal(Cell::default(), Literal::Atom(atom!("true")));
                self.payload.clause_clauses.push((head, body));
            }
            _ => {
                return Err(CompilationError::InadmissibleFact);
            }
        }

        Ok(())
    }

    fn add_extensible_predicate_declaration(
        &mut self,
        compilation_target: CompilationTarget,
        name: Atom,
        arity: usize,
        flag_accessor: impl Fn(&mut LocalPredicateSkeleton) -> &mut bool,
        retraction_fn: impl Fn(CompilationTarget, PredicateKey) -> RetractionRecord,
    ) -> Result<(), SessionError> {
        let key = (name, arity);
        let mut throw_permission_error = false;

        match &compilation_target {
            CompilationTarget::User => {
                match self
                    .wam_prelude
                    .indices
                    .extensible_predicates
                    .get_mut(&key)
                {
                    Some(skeleton) => {
                        if !*flag_accessor(&mut skeleton.core) {
                            *flag_accessor(&mut skeleton.core) = true;

                            self.payload.retraction_info.push_record(retraction_fn(
                                compilation_target,
                                key,
                            ));
                        }
                    }
                    None => {
                        if self.payload.compilation_target == compilation_target {
                            let mut skeleton = PredicateSkeleton::new();
                            *flag_accessor(&mut skeleton.core) = true;

                            self.add_extensible_predicate(
                                key,
                                skeleton,
                                CompilationTarget::User,
                            );
                        } else {
                            throw_permission_error = true;
                        }
                    }
                }
            }
            CompilationTarget::Module(ref module_name) => {
                match self.wam_prelude.indices.modules.get_mut(module_name) {
                    Some(ref mut module) => match module.extensible_predicates.get_mut(&key) {
                        Some(ref mut skeleton) => {
                            if !*flag_accessor(&mut skeleton.core) {
                                *flag_accessor(&mut skeleton.core) = true;

                                self.payload.retraction_info.push_record(retraction_fn(
                                    compilation_target,
                                    key,
                                ));
                            }
                        }
                        None => {
                            if self.payload.compilation_target == compilation_target {
                                let mut skeleton = PredicateSkeleton::new();
                                *flag_accessor(&mut skeleton.core) = true;

                                self.add_extensible_predicate(
                                    key,
                                    skeleton,
                                    compilation_target,
                                );
                            } else {
                                throw_permission_error = true;
                            }
                        }
                    },
                    None => {
                        self.add_dynamically_generated_module(*module_name);

                        let mut skeleton = PredicateSkeleton::new();
                        *flag_accessor(&mut skeleton.core) = true;

                        self.add_extensible_predicate(
                            key,
                            skeleton,
                            compilation_target,
                        );
                    }
                }
            }
        }

        if !throw_permission_error {
            let listing_src_file_name = self.listing_src_file_name();
            let payload_compilation_target = self.payload.compilation_target;

            match payload_compilation_target {
                CompilationTarget::User => {
                    match self
                        .wam_prelude
                        .indices
                        .get_local_predicate_skeleton_mut(
                            payload_compilation_target,
                            compilation_target,
                            listing_src_file_name,
                            key,
                        ) {
                        Some(skeleton) => {
                            if !*flag_accessor(skeleton) {
                                *flag_accessor(skeleton) = true;
                            }
                        }
                        None => {
                            let mut skeleton = LocalPredicateSkeleton::new();
                            *flag_accessor(&mut skeleton) = true;

                            self.add_local_extensible_predicate(
                                compilation_target,
                                key,
                                skeleton,
                            );
                        }
                    }
                }
                CompilationTarget::Module(ref module_name) => {
                    match self.wam_prelude.indices.modules.get_mut(module_name) {
                        Some(module) => match module
                            .local_extensible_predicates
                            .get_mut(&(compilation_target, key))
                        {
                            Some(skeleton) => {
                                if !*flag_accessor(skeleton) {
                                    *flag_accessor(skeleton) = true;
                                }
                            }
                            None => {
                                let mut skeleton = LocalPredicateSkeleton::new();
                                *flag_accessor(&mut skeleton) = true;

                                self.add_local_extensible_predicate(
                                    compilation_target,
                                    key,
                                    skeleton,
                                );
                            }
                        },
                        None => {
                            self.add_dynamically_generated_module(*module_name);

                            let mut skeleton = LocalPredicateSkeleton::new();
                            *flag_accessor(&mut skeleton) = true;

                            self.add_local_extensible_predicate(
                                compilation_target,
                                key,
                                skeleton,
                            );
                        }
                    }
                }
            }

            self.fail_on_undefined(compilation_target, key);

            Ok(())
        } else {
            Err(SessionError::PredicateNotMultifileOrDiscontiguous(
                compilation_target,
                key,
            ))
        }
    }

    fn fail_on_undefined(&mut self, compilation_target: CompilationTarget, key: PredicateKey) {
        /*
         * DynamicUndefined isn't only applied to dynamic predicates
         * but to multifile and discontiguous predicates as well.
         */

        let code_index = self.get_or_insert_code_index(key, compilation_target);

        if code_index.is_undefined() {
            set_code_index(
                &mut self.payload.retraction_info,
                &compilation_target,
                key,
                code_index,
                IndexPtr::dynamic_undefined(),
            );
        }
    }

    fn add_discontiguous_predicate(
        &mut self,
        compilation_target: CompilationTarget,
        name: Atom,
        arity: usize,
    ) -> Result<(), SessionError> {
        let key = (name, arity);

        let predicate_info = self
            .wam_prelude
            .indices
            .get_predicate_skeleton(&self.payload.predicates.compilation_target, &key)
            .map(|skeleton| skeleton.predicate_info())
            .unwrap_or_default();

        self.retract_local_clauses(&key, predicate_info.is_dynamic);

        self.add_extensible_predicate_declaration(
            compilation_target,
            name,
            arity,
            |skeleton| &mut skeleton.is_discontiguous,
            RetractionRecord::AddedDiscontiguousPredicate,
        )
    }

    fn add_dynamic_predicate(
        &mut self,
        compilation_target: CompilationTarget,
        name: Atom,
        arity: usize,
    ) -> Result<(), SessionError> {
        self.add_extensible_predicate_declaration(
            compilation_target,
            name,
            arity,
            |skeleton| &mut skeleton.is_dynamic,
            RetractionRecord::AddedDynamicPredicate,
        )
    }

    fn add_multifile_predicate(
        &mut self,
        compilation_target: CompilationTarget,
        name: Atom,
        arity: usize,
    ) -> Result<(), SessionError> {
        self.add_extensible_predicate_declaration(
            compilation_target,
            name,
            arity,
            |skeleton| &mut skeleton.is_multifile,
            RetractionRecord::AddedMultifilePredicate,
        )
    }

    fn add_clause_clause_if_dynamic(&mut self, term: &Term) -> Result<(), SessionError> {
        if let Some(predicate_name) = ClauseInfo::name(term) {
            let arity = ClauseInfo::arity(term);
            let predicates_compilation_target = self.payload.predicates.compilation_target;

            let is_dynamic = self
                .wam_prelude
                .indices
                .get_predicate_skeleton(
                    &predicates_compilation_target,
                    &(predicate_name, arity),
                )
                .map(|skeleton| skeleton.core.is_dynamic)
                .unwrap_or(false);

            if is_dynamic {
                self.add_clause_clause(term.clone())?;
            }
        }

        Ok(())
    }

    pub(super) fn retract_local_clauses(&mut self, key: &PredicateKey, is_dynamic: bool) {
        let payload_compilation_target = self.payload.compilation_target;
        let predicates_compilation_target = self.payload.predicates.compilation_target;
        let listing_src_file_name = self.listing_src_file_name();

        let clause_locs = match self
            .wam_prelude
            .indices
            .get_local_predicate_skeleton_mut(
                payload_compilation_target,
                predicates_compilation_target,
                listing_src_file_name,
                *key,
            ) {
            Some(skeleton) if !skeleton.clause_clause_locs.is_empty() => {
                mem::replace(&mut skeleton.clause_clause_locs, VecDeque::new())
            }
            _ => return,
        };

        self.payload.retraction_info.push_record(
            RetractionRecord::RemovedLocalSkeletonClauseLocations(
                payload_compilation_target,
                predicates_compilation_target,
                *key,
                clause_locs.clone(),
            ),
        );

        self.retract_local_clauses_impl(
            predicates_compilation_target,
            *key,
            &clause_locs,
        );

        if is_dynamic {
            let clause_clause_compilation_target = match predicates_compilation_target {
                CompilationTarget::User => CompilationTarget::Module(atom!("builtins")),
                module_name => module_name,
            };

            self.retract_local_clause_clauses(clause_clause_compilation_target, &clause_locs);
        }
    }
}

impl<'a> MachinePreludeView<'a> {
    #[inline]
    pub(super) fn composite_op_dir(&self, compilation_target: &CompilationTarget) -> CompositeOpDir {
        match compilation_target {
            CompilationTarget::User => CompositeOpDir::new(&self.indices.op_dir, None),
            CompilationTarget::Module(ref module_name) => {
                match self.indices.modules.get(module_name) {
                    Some(ref module) => {
                        CompositeOpDir::new(&self.indices.op_dir, Some(&module.op_dir))
                    }
                    None => {
                        unreachable!()
                    }
                }
            }
        }
    }
}

impl MachineState {
    pub(super) fn read_term_from_heap(&mut self, term_addr: HeapCellValue) -> Result<Term, SessionError> {
        let mut term_stack = vec![];
        let mut iter = stackful_post_order_iter(&mut self.heap, &mut self.stack, term_addr);

        while let Some(addr) = iter.next() {
            let addr = unmark_cell_bits!(addr);

            read_heap_cell!(addr,
                (HeapCellValueTag::Lis) => {
                    use crate::parser::parser::as_partial_string;

                    let tail = term_stack.pop().unwrap();
                    let head = term_stack.pop().unwrap();

                    match as_partial_string(head, tail) {
                        Ok((string, Some(tail))) => {
                            term_stack.push(Term::PartialString(Cell::default(), string, tail));
                        }
                        Ok((string, None)) => {
                            let atom = self.atom_tbl.build_with(&string);
                            term_stack.push(Term::CompleteString(Cell::default(), atom));
                        }
                        Err(cons_term) => term_stack.push(cons_term),
                    }
                }
                (HeapCellValueTag::Var | HeapCellValueTag::AttrVar | HeapCellValueTag::StackVar, h) => {
                    term_stack.push(Term::Var(Cell::default(), VarPtr::from(format!("_{}", h))));
                }
                (HeapCellValueTag::Cons | HeapCellValueTag::CStr | HeapCellValueTag::Fixnum |
                 HeapCellValueTag::Char | HeapCellValueTag::F64) => {
                    term_stack.push(Term::Literal(Cell::default(), Literal::try_from(addr).unwrap()));
                }
                (HeapCellValueTag::Atom, (name, arity)) => {
                    let h = iter.focus().value() as usize;
                    let mut arity = arity;

                    if iter.heap.len() > h + arity + 1 {
                        let value = iter.heap[h + arity + 1];

                        if let Some(idx) = get_structure_index(value) {
                            // in the second condition, arity == 0,
                            // meaning idx cannot pertain to this atom
                            // if it is the direct subterm of a larger
                            // structure.
                            if arity > 0 || !iter.direct_subterm_of_str(h) {
                                term_stack.push(
                                    Term::Literal(Cell::default(), Literal::CodeIndex(idx))
                                );

                                arity += 1;
                            }
                        }
                    }

                    if arity == 0 {
                        term_stack.push(Term::Literal(Cell::default(), Literal::Atom(name)));
                    } else {
                        let subterms = term_stack
                            .drain(term_stack.len() - arity ..)
                            .collect();

                        term_stack.push(Term::Clause(Cell::default(), name, subterms));
                    }
                }
                (HeapCellValueTag::PStr, atom) => {
                    let tail = term_stack.pop().unwrap();

                    if let Term::Literal(_, Literal::Atom(atom!("[]"))) = &tail {
                        term_stack.push(Term::CompleteString(Cell::default(), atom));
                    } else {
                        term_stack.push(Term::PartialString(
                            Cell::default(),
                            atom.as_str().to_owned(),
                            Box::new(tail),
                        ));
                    }
                }
                (HeapCellValueTag::PStrLoc, h) => {
                    let atom = cell_as_atom_cell!(iter.heap[h]).get_name();
                    let tail = term_stack.pop().unwrap();

                    term_stack.push(Term::PartialString(
                        Cell::default(),
                        atom.as_str().to_owned(),
                        Box::new(tail),
                    ));
                }
                _ => {
                }
            );
        }

        debug_assert!(term_stack.len() == 1);
        Ok(term_stack.pop().unwrap())
    }
}

impl Machine {
    pub(crate) fn use_module(&mut self) -> CallResult {
        let subevacuable_addr = self
            .machine_st
            .store(self.machine_st.deref(self.machine_st.registers[2]));

        let module_src = ModuleSource::Library({
            let payload = cell_as_load_state_payload!(subevacuable_addr);

            match payload.compilation_target {
                CompilationTarget::Module(module_name) => module_name,
                CompilationTarget::User => {
                    return Ok(());
                }
            }
        });

        let mut loader = self.loader_from_heap_evacuable(temp_v!(1));

        let use_module = || {
            let export_list = loader.extract_module_export_list_from_heap(temp_v!(3))?;

            if export_list.is_empty() {
                loader.use_module(module_src)?;
            } else {
                loader.use_qualified_module(module_src, export_list)?;
            }

            LiveLoadAndMachineState::evacuate(loader)
        };

        let result = use_module();
        self.restore_load_state_payload(result)
    }

    pub(crate) fn load_compiled_library(&mut self) -> CallResult {
        let library = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        if let Some(module) = self.indices.modules.get(&library) {
            if let ListingSource::DynamicallyGenerated = module.listing_src {
                self.machine_st.fail = true;
                return Ok(());
            }

            let mut loader = self.loader_from_heap_evacuable(temp_v!(3));

            let import_module = || {
                let export_list = loader.extract_module_export_list_from_heap(temp_v!(2))?;

                if export_list.is_empty() {
                    loader.import_module(library)?;
                } else {
                    loader.import_qualified_module(library, export_list)?;
                }

                LiveLoadAndMachineState::evacuate(loader)
            };

            let result = import_module();
            self.restore_load_state_payload(result)
        } else {
            self.machine_st.fail = true;
            Ok(())
        }
    }

    pub(crate) fn declare_module(&mut self) -> CallResult {
        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let mut loader = self.loader_from_heap_evacuable(temp_v!(3));

        let declare_module = || {
            let exports = loader.extract_module_export_list_from_heap(temp_v!(2))?;

            let module_decl = ModuleDecl {
                name: module_name,
                exports: exports.into_iter().collect(),
            };

            loader.load_decl(Declaration::Module(module_decl))?;
            LiveLoadAndMachineState::evacuate(loader)
        };

        let result = declare_module();
        self.restore_load_state_payload(result)
    }

    #[inline]
    pub(crate) fn add_discontiguous_predicate(&mut self) -> CallResult {
        self.add_extensible_predicate_declaration(
            |loader, compilation_target, clause_name, arity| {
                loader.add_discontiguous_predicate(compilation_target, clause_name, arity)
            },
        )
    }

    #[inline]
    pub(crate) fn add_dynamic_predicate(&mut self) -> CallResult {
        self.add_extensible_predicate_declaration(
            |loader, compilation_target, clause_name, arity| {
                loader.add_dynamic_predicate(compilation_target, clause_name, arity)
            },
        )
    }

    #[inline]
    pub(crate) fn add_multifile_predicate(&mut self) -> CallResult {
        self.add_extensible_predicate_declaration(
            |loader, compilation_target, clause_name, arity| {
                loader.add_multifile_predicate(compilation_target, clause_name, arity)
            },
        )
    }

    fn add_extensible_predicate_declaration(
        &mut self,
        decl_adder: impl for<'a> Fn(
            &mut Loader<'a, LiveLoadAndMachineState<'a>>,
            CompilationTarget,
            Atom,
            usize,
        ) -> Result<(), SessionError>,
    ) -> CallResult {
        let module_name = cell_as_atom!(self.deref_register(1));

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        let predicate_name = cell_as_atom!(self.deref_register(2));

        let arity = self.deref_register(3);
        let arity = match Number::try_from(arity) {
            Ok(Number::Integer(n)) if &*n >= &Integer::from(0) && &*n <= &Integer::from(MAX_ARITY) => Ok(n.to_usize().unwrap()),
            Ok(Number::Fixnum(n)) if n.get_num() >= 0 && n.get_num() <= MAX_ARITY as i64 => {
                Ok(usize::try_from(n.get_num()).unwrap())
            }
            _ => Err(SessionError::from(CompilationError::InvalidRuleHead)),
        };

        let mut loader = self.loader_from_heap_evacuable(temp_v!(4));

        let add_predicate_decl = move || {
            decl_adder(&mut loader, compilation_target, predicate_name, arity?)?;
            LiveLoadAndMachineState::evacuate(loader)
        };

        let result = add_predicate_decl();
        self.restore_load_state_payload(result)
    }

    pub(crate) fn add_term_expansion_clause(&mut self) -> CallResult {
        let mut loader = self.loader_from_heap_evacuable(temp_v!(2));

        let add_clause = || {
            let term = loader.read_term_from_heap(temp_v!(1))?;

            loader.incremental_compile_clause(
                (atom!("term_expansion"), 2),
                term,
                CompilationTarget::User,
                false,
                AppendOrPrepend::Append,
            )?;

            LiveLoadAndMachineState::evacuate(loader)
        };

        let result = add_clause();
        self.restore_load_state_payload(result)
    }

    pub(crate) fn add_goal_expansion_clause(&mut self) -> CallResult {
        let target_module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let mut loader = self.loader_from_heap_evacuable(temp_v!(3));

        let compilation_target = match target_module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(target_module_name),
        };

        let add_clause = || {
            let term = loader.read_term_from_heap(temp_v!(2))?;

            let indexing_arg = match term.name() {
                Some(atom!(":-")) => term.first_arg().and_then(Term::first_arg),
                Some(_) => term.first_arg(),
                None => None,
            };

            if let Some(indexing_term) = indexing_arg {
                if let Some(indexing_name) = indexing_term.name() {
                    loader.wam_prelude
                        .indices
                        .goal_expansion_indices
                        .insert((indexing_name, indexing_term.arity()));
                }
            }

            loader.incremental_compile_clause(
                (atom!("goal_expansion"), 2),
                term,
                compilation_target,
                false, // backtracking inferences are counted by call_with_inference_limit.
                AppendOrPrepend::Append,
            )?;

            LiveLoadAndMachineState::evacuate(loader)
        };

        let result = add_clause();
        self.restore_load_state_payload(result)
    }

    pub(crate) fn add_in_situ_filename_module(&mut self) -> CallResult {
        let mut loader = self.loader_from_heap_evacuable(temp_v!(1));

        let add_in_situ_filename_module = || {
            if let Some(filename) = loader.listing_src_file_name() {
                let module_decl = ModuleDecl {
                    name: filename,
                    exports: vec![],
                };

                let module_name = module_decl.name;

                if !loader
                    .wam_prelude
                    .indices
                    .modules
                    .contains_key(&module_decl.name)
                {
                    let module = Module::new_in_situ(module_decl);
                    loader
                        .wam_prelude
                        .indices
                        .modules
                        .insert(module_name, module);
                } else {
                    loader.reset_in_situ_module(
                        module_decl.clone(),
                        &ListingSource::DynamicallyGenerated,
                    );

                    match loader.wam_prelude.indices.modules.get_mut(&module_name) {
                        Some(module) => {
                            for (key, value) in module.op_dir.drain(0..) {
                                let mut op_decl = OpDecl::new(value, key.0);
                                op_decl.remove(&mut loader.wam_prelude.indices.op_dir);
                            }
                        }
                        None => {}
                    }
                }
            }

            LiveLoadAndMachineState::evacuate(loader)
        };

        let result = add_in_situ_filename_module();
        self.restore_load_state_payload(result)
    }

    pub(crate) fn loader_from_heap_evacuable<'a>(
        &'a mut self,
        r: RegType,
    ) -> Loader<'a, LiveLoadAndMachineState<'a>> {
        let mut load_state = cell_as_load_state_payload!(
            self.machine_st.store(self.machine_st.deref(self.machine_st[r]))
        );

        load_state.set_tag(ArenaHeaderTag::LiveLoadState);

        let (wam_prelude, machine_st) = self.prelude_view_and_machine_st();

        Loader {
            payload: LiveLoadAndMachineState { load_state, machine_st },
            wam_prelude,
        }
    }

    #[inline]
    pub(crate) fn push_load_state_payload(&mut self) {
        let payload = arena_alloc!(
            LoadStatePayload::new(
                self.code.len(),
                LiveTermStream::new(ListingSource::User),
            ),
            &mut self.machine_st.arena
        );

        let var = self.machine_st.deref(self.machine_st.registers[1]);

        self.machine_st.bind(
            var.as_var().unwrap(),
            typed_arena_ptr_as_cell!(payload),
        );
    }

    #[inline]
    pub(crate) fn pop_load_state_payload(&mut self) {
        let load_state_payload = self.machine_st.store(
            self.machine_st.deref(self.machine_st.registers[1])
        );

        // unlike in loader_from_heap_evacuable,
        // pop_load_state_payload is allowed to fail to find a
        // LoadStatePayload in the heap, as a Rust-side
        // top-level command may have failed to write the
        // load state payload back to the heap.

        read_heap_cell!(load_state_payload,
            (HeapCellValueTag::Cons, cons_ptr) => {
                match_untyped_arena_ptr!(cons_ptr,
                    (ArenaHeaderTag::LiveLoadState, payload) => {
                        unsafe {
                            std::ptr::drop_in_place(
                                payload.as_ptr() as *mut LiveLoadState,
                            );
                        }
                    }
                    _ => {}
                );
            }
            _ => {}
        );
    }

    #[inline]
    pub(crate) fn pop_load_context(&mut self) {
        self.load_contexts.pop();
    }

    pub(crate) fn push_load_context(&mut self) -> CallResult {
        let stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("$push_load_context"),
            2,
        )?;

        let path = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[2]))
        );

        self.load_contexts.push(LoadContext::new(path.as_str(), stream));
        Ok(())
    }

    pub(crate) fn restore_load_state_payload(
        &mut self,
        result: Result<TypedArenaPtr<LiveLoadState>, SessionError>,
    ) -> CallResult {
        match result {
            Ok(_payload) => {
                Ok(())
            }
            Err(e) => {
                let err = self.machine_st.session_error(e);
                let stub = functor_stub(atom!("load"), 1);

                Err(self.machine_st.error_form(err, stub))
            }
        }
    }

    pub(crate) fn scoped_clause_to_evacuable(&mut self) -> CallResult {
        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let loader = self.loader_from_heap_evacuable(temp_v!(3));

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        let result = loader.read_and_enqueue_term(temp_v!(2), compilation_target);
        self.restore_load_state_payload(result)
    }

    pub(crate) fn clause_to_evacuable(&mut self) -> CallResult {
        let loader = self.loader_from_heap_evacuable(temp_v!(2));
        let compilation_target = loader.payload.compilation_target;

        let result = loader.read_and_enqueue_term(temp_v!(1), compilation_target);
        self.restore_load_state_payload(result)
    }

    pub(crate) fn conclude_load(&mut self) -> CallResult {
        let mut loader = self.loader_from_heap_evacuable(temp_v!(1));

        let compile_final_terms = || {
            if !loader.payload.predicates.is_empty() {
                loader.compile_and_submit()?;
            }

            loader.remove_module_op_exports();
            LiveLoadAndMachineState::evacuate(loader)
        };

        let result = compile_final_terms();
        self.restore_load_state_payload(result)
    }

    pub(crate) fn load_context_source(&mut self) {
        if let Some(load_context) = self.load_contexts.last() {
            let path_str = load_context.path.to_str().unwrap();
            let path_atom = self.machine_st.atom_tbl.build_with(path_str);

            self.machine_st.unify_atom(path_atom, self.machine_st.registers[1]);
        } else {
            self.machine_st.fail = true;
        }
    }

    pub(crate) fn load_context_file(&mut self) {
        if let Some(load_context) = self.load_contexts.last() {
            match load_context.path.file_name() {
                Some(file_name) if load_context.path.is_file() => {
                    let file_name_str = file_name.to_str().unwrap();
                    let file_name_atom = self.machine_st.atom_tbl.build_with(file_name_str);

                    self.machine_st.unify_atom(file_name_atom, self.machine_st.registers[1]);
                    return;
                }
                _ => {
                    return self.load_context_module(self.machine_st.registers[1]);
                }
            }
        }

        self.machine_st.fail = true;
    }

    pub(crate) fn load_context_directory(&mut self) {
        if let Some(load_context) = self.load_contexts.last() {
            if let Some(directory) = load_context.path.parent() {
                let directory_str = directory.to_str().unwrap();
                let directory_atom = self.machine_st.atom_tbl.build_with(directory_str);

                self.machine_st.unify_atom(directory_atom, self.machine_st.registers[1]);
                return;
            }
        }

        self.machine_st.fail = true;
    }

    pub(crate) fn load_context_module(&mut self, target: HeapCellValue) {
        if let Some(load_context) = self.load_contexts.last() {
            self.machine_st.unify_atom(load_context.module, target);
        } else {
            self.machine_st.fail = true;
        }
    }

    pub(crate) fn load_context_stream(&mut self) {
        if let Some(load_context) = self.load_contexts.last() {
            self.machine_st.unify_constant(
                UntypedArenaPtr::from(load_context.stream.as_ptr()),
                self.machine_st.registers[1],
            );
        } else {
            self.machine_st.fail = true;
        }
    }

    pub(crate) fn compile_assert(&mut self, append_or_prepend: AppendOrPrepend) -> CallResult {
        let module_name = cell_as_atom!(self.deref_register(1));

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        let stub_gen = || {
            match append_or_prepend {
                AppendOrPrepend::Append  => functor_stub(atom!("assertz"), 1),
                AppendOrPrepend::Prepend => functor_stub(atom!("asserta"), 1),
            }
        };

        let head = self.deref_register(2);

        if head.is_var() {
            let err = self.machine_st.instantiation_error();
            return Err(self.machine_st.error_form(err, stub_gen()));
        }

        let mut compile_assert = || {
            let mut loader: Loader<'_, LiveLoadAndMachineState<'_>> =
                Loader::new(self, LiveTermStream::new(ListingSource::User));

            loader.payload.compilation_target = compilation_target;

            let head = LiveLoadAndMachineState::machine_st(&mut loader.payload).read_term_from_heap(head)?;

            let name = if let Some(name) = head.name() {
                name
            } else {
                return Err(SessionError::from(CompilationError::InvalidRuleHead));
            };

            let arity = head.arity();
            let is_builtin = loader.wam_prelude.indices.builtin_property((name, arity));

            let is_dynamic_predicate = loader
                  .wam_prelude
                  .indices
                  .is_dynamic_predicate(
                      module_name,
                      (name, arity),
                  );

            let no_such_predicate =
                if !is_dynamic_predicate && !is_builtin {
                    let idx_tag = loader
                        .wam_prelude
                        .indices
                        .get_predicate_code_index(
                            name,
                            arity,
                            module_name,
                        )
                        .map(|code_idx| code_idx.get_tag())
                        .unwrap_or(IndexPtrTag::DynamicUndefined);

                    idx_tag == IndexPtrTag::DynamicUndefined || idx_tag == IndexPtrTag::Undefined
                } else if is_builtin {
                    return Err(SessionError::CannotOverwriteBuiltIn((name, arity)));
                } else {
                    is_dynamic_predicate
                };

            if !no_such_predicate {
                LiveLoadAndMachineState::machine_st(&mut loader.payload).fail = true;
                return LiveLoadAndMachineState::evacuate(loader);
            }

            let body = loader.read_term_from_heap(temp_v!(3))?;

            let asserted_clause = Term::Clause(
                Cell::default(),
                atom!(":-"),
                vec![head.clone(), body.clone()],
            );

            // if a new predicate was just created, make it dynamic.
            loader.add_dynamic_predicate(compilation_target, name, arity)?;

            loader.incremental_compile_clause(
                (name, arity),
                asserted_clause,
                compilation_target,
                false,
                append_or_prepend,
            )?;

            // the global clock is incremented after each assertion.
            LiveLoadAndMachineState::machine_st(&mut loader.payload).global_clock += 1;

            loader.compile_clause_clauses(
                (name, arity),
                compilation_target,
                std::iter::once((head, body)),
                append_or_prepend,
            )?;

            LiveLoadAndMachineState::evacuate(loader)
        };

        match compile_assert() {
            Ok(_) => Ok(()),
            Err(SessionError::CompilationError(
                CompilationError::InvalidRuleHead |
                CompilationError::InadmissibleFact
            )) => {
                let err = self.machine_st.type_error(
                    ValidType::Callable,
                    self.machine_st.registers[2],
                );

                Err(self.machine_st.error_form(err, stub_gen()))
            }
            Err(SessionError::CompilationError(
                CompilationError::InadmissibleQueryTerm
            )) => {
                let err = self.machine_st.type_error(
                    ValidType::Callable,
                    self.machine_st.registers[3],
                );

                Err(self.machine_st.error_form(err, stub_gen()))
            }
            Err(e) => {
                let err = self.machine_st.session_error(e);
                Err(self.machine_st.error_form(err, stub_gen()))
            }
        }
    }

    pub(crate) fn abolish_clause(&mut self) -> CallResult {
        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let key = self
            .machine_st
            .read_predicate_key(self.machine_st.registers[2], self.machine_st.registers[3]);

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        let mut abolish_clause = || {
            let mut loader: Loader<'_, LiveLoadAndMachineState<'_>>
                = Loader::new(self, LiveTermStream::new(ListingSource::User));

            loader.payload.compilation_target = compilation_target;

            let clause_clause_compilation_target = match compilation_target {
                CompilationTarget::User => CompilationTarget::Module(atom!("builtins")),
                module => module,
            };

            let mut clause_clause_target_poses: Vec<_> = loader
                .wam_prelude
                .indices
                .remove_predicate_skeleton(&compilation_target, &key)
                .map(|skeleton| {
                    let mut clause_clause_skeleton = loader
                        .wam_prelude
                        .indices
                        .remove_predicate_skeleton(
                            &clause_clause_compilation_target,
                            &(atom!("$clause"), 2),
                        ).unwrap();

                    let result = skeleton.core
                        .clause_clause_locs
                        .iter()
                        .map(|clause_clause_loc| {
                            clause_clause_skeleton
                                .target_pos_of_clause_clause_loc(*clause_clause_loc)
                                .unwrap()
                        })
                        .collect();

                    loader.add_extensible_predicate(
                        key,
                        skeleton,
                        compilation_target,
                    );

                    loader.add_extensible_predicate(
                        (atom!("$clause"), 2),
                        clause_clause_skeleton,
                        clause_clause_compilation_target,
                    );

                    result
                }).unwrap();

            loader.wam_prelude
                .indices
                .remove_predicate_skeleton(&compilation_target, &key);

            let mut code_index = loader
                .get_or_insert_code_index(key, compilation_target);

            code_index.set(IndexPtr::undefined());

            loader.payload.compilation_target = clause_clause_compilation_target;

            while let Some(target_pos) = clause_clause_target_poses.pop() {
                loader.retract_clause((atom!("$clause"), 2), target_pos);
            }

            LiveLoadAndMachineState::evacuate(loader)
        };

        match abolish_clause() {
            Ok(_) => Ok(()),
            Err(e) => {
                let stub = functor_stub(atom!("abolish"), 1);
                let err = self.machine_st.session_error(e);
                Err(self.machine_st.error_form(err, stub))
            }
        }
    }

    pub(crate) fn retract_clause(&mut self) -> CallResult {
        let key = self
            .machine_st
            .read_predicate_key(self.machine_st[temp_v!(1)], self.machine_st[temp_v!(2)]);

        let target_pos = self
            .machine_st
            .store(self.machine_st.deref(self.machine_st[temp_v!(3)]));

        let target_pos = match Number::try_from(target_pos) {
            Ok(Number::Integer(n)) => n.to_usize().unwrap(),
            Ok(Number::Fixnum(n)) => usize::try_from(n.get_num()).unwrap(),
            _ => unreachable!(),
        };

        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[4]))
        );

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        let clause_clause_compilation_target = match compilation_target {
            CompilationTarget::User => CompilationTarget::Module(atom!("builtins")),
            _ => compilation_target,
        };

        let mut retract_clause = || {
            let mut loader: Loader<'_, LiveLoadAndMachineState<'_>> =
                Loader::new(self, LiveTermStream::new(ListingSource::User));

            loader.payload.compilation_target = compilation_target;

            let clause_clause_loc = loader.retract_dynamic_clause(key, target_pos);

            // the global clock is incremented after each retraction.
            LiveLoadAndMachineState::machine_st(&mut loader.payload).global_clock += 1;

            let target_pos = match loader.wam_prelude.indices.get_predicate_skeleton_mut(
                &clause_clause_compilation_target,
                &(atom!("$clause"), 2),
            ) {
                Some(skeleton) => skeleton
                    .target_pos_of_clause_clause_loc(clause_clause_loc)
                    .unwrap(),
                None => {
                    unreachable!();
                }
            };

            loader.payload.compilation_target = clause_clause_compilation_target;
            loader.retract_clause((atom!("$clause"), 2), target_pos);

            LiveLoadAndMachineState::evacuate(loader)
        };

        match retract_clause() {
            Ok(_) => Ok(()),
            Err(e) => {
                let stub = functor_stub(atom!("retract"), 1);
                let err = self.machine_st.session_error(e);

                Err(self.machine_st.error_form(err, stub))
            }
        }
    }

    pub(crate) fn is_consistent_with_term_queue(&mut self) -> CallResult {
        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let key = self
            .machine_st
            .read_predicate_key(self.machine_st[temp_v!(2)], self.machine_st[temp_v!(3)]);

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        let mut loader = self.loader_from_heap_evacuable(temp_v!(4));

        LiveLoadAndMachineState::machine_st(&mut loader.payload).fail =
            (!loader.payload.predicates.is_empty()
             && loader.payload.predicates.compilation_target != compilation_target)
             || !key.is_consistent(&loader.payload.predicates);

        let result = LiveLoadAndMachineState::evacuate(loader);
        self.restore_load_state_payload(result)
    }

    pub(crate) fn flush_term_queue(&mut self) -> CallResult {
        let mut loader = self.loader_from_heap_evacuable(temp_v!(1));

        let flush_term_queue = || {
            if !loader.payload.predicates.is_empty() {
                loader.compile_and_submit()?;
            }

            LiveLoadAndMachineState::evacuate(loader)
        };

        let result = flush_term_queue();
        self.restore_load_state_payload(result)
    }

    pub(crate) fn remove_module_exports(&mut self) -> CallResult {
        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let mut loader = self.loader_from_heap_evacuable(temp_v!(2));

        let remove_module_exports = || {
            loader.remove_module_exports(module_name);
            LiveLoadAndMachineState::evacuate(loader)
        };

        let result = remove_module_exports();
        self.restore_load_state_payload(result)
    }

    pub(crate) fn add_non_counted_backtracking(&mut self) -> CallResult {
        let key = self
            .machine_st
            .read_predicate_key(self.machine_st[temp_v!(1)], self.machine_st[temp_v!(2)]);

        let mut loader = self.loader_from_heap_evacuable(temp_v!(3));
        loader.payload.non_counted_bt_preds.insert(key);

        let result = LiveLoadAndMachineState::evacuate(loader);
        self.restore_load_state_payload(result)
    }

    pub(crate) fn meta_predicate_property(&mut self) {
        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let (predicate_name, arity) = self
            .machine_st
            .read_predicate_key(self.machine_st.registers[2], self.machine_st.registers[3]);

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        match self
            .indices
            .get_meta_predicate_spec(predicate_name, arity, &compilation_target)
        {
            Some(meta_specs) => {
                let term_loc = self.machine_st.heap.len();

                self.machine_st.heap.push(atom_as_cell!(predicate_name, arity));
                self.machine_st.heap.extend(
                    meta_specs.iter().map(|meta_spec| match meta_spec {
                        MetaSpec::Minus => atom_as_cell!(atom!("+")),
                        MetaSpec::Plus => atom_as_cell!(atom!("-")),
                        MetaSpec::Either => atom_as_cell!(atom!("?")),
                        MetaSpec::Colon => atom_as_cell!(atom!(":")),
                        MetaSpec::RequiresExpansionWithArgument(ref arg_num) => {
                            fixnum_as_cell!(Fixnum::build_with(*arg_num as i64))
                        }
                    })
                );

                let heap_loc = self.machine_st.heap.len();

                self.machine_st.heap.push(atom_as_cell!(atom!("meta_predicate"), 1));
                self.machine_st.heap.push(str_loc_as_cell!(term_loc));

                unify!(self.machine_st, str_loc_as_cell!(heap_loc), self.machine_st.registers[4]);
            }
            None => {
                self.machine_st.fail = true;
            }
        }
    }

    pub(crate) fn dynamic_property(&mut self) {
        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let key = self
            .machine_st
            .read_predicate_key(self.machine_st[temp_v!(2)], self.machine_st[temp_v!(3)]);

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        match self
            .indices
            .get_predicate_skeleton(&compilation_target, &key)
        {
            Some(skeleton) => {
                self.machine_st.fail = !skeleton.core.is_dynamic;
            }
            None => {
                self.machine_st.fail = true;
            }
        }
    }

    pub(crate) fn multifile_property(&mut self) {
        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let key = self
            .machine_st
            .read_predicate_key(self.machine_st[temp_v!(2)], self.machine_st[temp_v!(3)]);

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        match self
            .indices
            .get_predicate_skeleton(&compilation_target, &key)
        {
            Some(skeleton) => {
                self.machine_st.fail = !skeleton.core.is_multifile;
            }
            None => {
                self.machine_st.fail = true;
            }
        }
    }

    pub(crate) fn discontiguous_property(&mut self) {
        let module_name = cell_as_atom!(
            self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]))
        );

        let key = self
            .machine_st
            .read_predicate_key(self.machine_st[temp_v!(2)], self.machine_st[temp_v!(3)]);

        let compilation_target = match module_name {
            atom!("user") => CompilationTarget::User,
            _ => CompilationTarget::Module(module_name),
        };

        match self
            .indices
            .get_predicate_skeleton(&compilation_target, &key)
        {
            Some(skeleton) => {
                self.machine_st.fail = !skeleton.core.is_discontiguous;
            }
            None => {
                self.machine_st.fail = true;
            }
        }
    }
}

impl<'a> Loader<'a, LiveLoadAndMachineState<'a>> {
    fn read_and_enqueue_term(
        mut self,
        term_reg: RegType,
        compilation_target: CompilationTarget,
    ) -> Result<TypedArenaPtr<LoadStatePayload<LiveTermStream>>, SessionError> {
        if self.payload.predicates.compilation_target != compilation_target {
            if !self.payload.predicates.is_empty() {
                self.compile_and_submit()?;
            }

            self.payload.predicates.compilation_target = compilation_target;
        }

        let term = self.read_term_from_heap(term_reg)?;

        self.add_clause_clause_if_dynamic(&term)?;
        self.payload.term_stream.term_queue.push_back(term);

        self.load()
    }
}

#[inline]
pub(super) fn load_module(
    machine_st: &mut MachineState,
    code_dir: &mut CodeDir,
    op_dir: &mut OpDir,
    meta_predicate_dir: &mut MetaPredicateDir,
    compilation_target: &CompilationTarget,
    module: &Module,
) {
    let ts = LiveTermStream::new(ListingSource::User);
    let payload = LoadStatePayload::new(0, ts);
    let mut payload = LiveLoadAndMachineState::new(machine_st, payload);

    import_module_exports::<LiveLoadAndMachineState>(
        &mut payload,
        &compilation_target,
        module,
        code_dir,
        op_dir,
        meta_predicate_dir,
    )
    .unwrap();
}