src/Pure/Syntax/ast.ML

--- a/src/Pure/Syntax/ast.ML	Tue May 17 10:19:43 2005 +0200
+++ b/src/Pure/Syntax/ast.ML	Tue May 17 10:19:44 2005 +0200
@@ -210,7 +210,7 @@
     val changes = ref 0;
 
     fun subst _ (ast as Constant _) = ast
-      | subst env (Variable x) = valOf (Symtab.lookup (env, x))
+      | subst env (Variable x) = the (Symtab.lookup (env, x))
       | subst env (Appl asts) = Appl (map (subst env) asts);
 
     fun try_rules ((lhs, rhs) :: pats) ast =