group predicate subsequences by first instantiated arg (#1053)

src/forms.rs

@@ -219,7 +219,6 @@ pub enum PredicateClause {
 }
 
 impl PredicateClause {
-    // TODO: add this to `Term` in `crate::parser` like `first_arg`.
     pub(crate) fn args(&self) -> Option<&[Term]> {
         match self {
             PredicateClause::Fact(term, ..) => match term {
@@ -237,6 +236,13 @@ impl PredicateClause {
     }
 }
 
+#[derive(Debug)]
+pub struct ClauseSpan {
+    pub left: usize,
+    pub right: usize,
+    pub instantiated_arg_index: usize,
+}
+
 #[derive(Debug, Clone)]
 pub enum ModuleSource {
     Library(Atom),