streamline code generation with iterators and traits

src/l0/l0_parser.lalrpop

@@ -1,3 +1,4 @@
+use std::cell::{Cell};
 use l0::ast::{Atom, Term, TopLevel, Var};
 
 grammar;
@@ -23,8 +24,8 @@ Term : Term = {
      <a:Atom> "(" <ts: (<BoxedTerm> ",")*> <t:BoxedTerm> ")" => {
      	      let mut ts = ts;
      	      ts.push(t);
-	      Term::Clause(a, ts)
+	      Term::Clause(Cell::new(0), a, ts)
      },
-     <Atom> => Term::Atom(<>),
-     <Var>  => Term::Var(<>),
+     <Atom> => Term::Atom(Cell::new(0), <>),
+     <Var>  => Term::Var(Cell::new(0), <>),
 };