fix bug with mark_non_var

README.md

@@ -35,7 +35,7 @@ Extend rusty-wam to include the following, among other features:
     - A representation of 'partial strings' as difference lists
       of characters (_done_).
 * `term_expansion/2` and `goal_expansion/2` (_in progress_).
-* Definite Clause Grammars.
+* Definite Clause Grammars (_in progress_).
 * Attributed variables using the SICStus Prolog interface and
   semantics. Adding coroutines like `dif/2`, `freeze/2`, etc.
   is straightforward with attributed variables.

src/prolog/debray_allocator.rs

@@ -232,7 +232,6 @@ impl<'a> Allocator<'a> for DebrayAllocator
     {
         let r = cell.get();
         
-        if r.reg_num() == 0 {
         let r = match lvl {
             Level::Shallow => {
                 let k = self.arg_c;
@@ -244,12 +243,12 @@ impl<'a> Allocator<'a> for DebrayAllocator
                 self.arg_c += 1;
                 RegType::Temp(k)
             },
-                _ => RegType::Temp(self.alloc_reg_to_non_var())
+            _ if r.reg_num() == 0 => RegType::Temp(self.alloc_reg_to_non_var()),
+            _ => r
         };
 
         cell.set(r);        
     }
-    }
 
     fn mark_var<Target>(&mut self, var: Rc<Var>, lvl: Level, cell: &'a Cell<VarReg>,
                         term_loc: GenContext, target: &mut Vec<Target>)

src/prolog/lib/dcgs.pl

@@ -25,7 +25,7 @@ term_expansion(Term0, (ModHead :- ModBody)) :-
     !,
     nonvar(Head),
     Head =.. [RuleName | Args],
-    append(Args, ['$VAR'(0), '$VAR'(N)], ModArgs),
+    append(Args, ['$VAR'(0), '$VAR'(N)], ModArgs), %% problematic line.
     ModHead =.. [RuleName | ModArgs],
     nonvar(Body),
     expand_body(Body, ModBody1, 0, N1),