[sledge] Fix Context.apply_subst to preserve rest of representation

Summary:
Context.apply_subst erroneously resets everything in the
representation of a context except the solution substitution when
applying a substitution.

Reviewed By: jvillard

Differential Revision: D25756548

fbshipit-source-id: 949a34ced

sledge/src/fol/context.ml

@@ -683,7 +683,8 @@ let apply_subst wrt s r =
   ;
   ( if Subst.is_empty s then r
   else
-    Trm.Map.fold (classes r) empty ~f:(fun ~key:rep ~data:cls r ->
+    Trm.Map.fold (classes r) {r with rep= Subst.empty}
+      ~f:(fun ~key:rep ~data:cls r ->
         let rep' = Subst.subst_ s rep in
         List.fold cls r ~f:(fun trm r ->
             let trm' = Subst.subst_ s trm in