Improve the heuristic for performing substitutions in the Gappa driver.

The original idea was to replace t1 by t2 only when t1 is more complicated
than t2, which was implemented by saying that t1 is neither a variable nor
a constant. Unfortunately, it means that no substitution happens when both
t1 and t2 are variables and/or constants. This patch fixes it.

src/printer/gappa.ml

@@ -44,8 +44,13 @@ let () =
   Trans.register_transform "simplify_unknown_lsymbols"
     (syntactic_transform (fun check_ls -> Trans.goal (fun pr f ->
       [create_prop_decl Pgoal pr (Simplify_formula.fmla_cond_subst
-        (fun t _ -> match t.t_node with
-          | Tconst _ | Tapp(_,[]) -> false
+        (fun t1 t2 -> match t1.t_node with
+          | Tconst _ -> false
+          | Tapp(_,[]) ->
+            begin match t2.t_node with
+            | Tconst _ | Tapp(_,[]) -> true
+            | _ -> false
+            end
           | Tapp(ls,_) -> not (check_ls ls)
           | _ -> true) f)
       ])))