reverseFs :: Type FieldSelector -> Typing Type
reverseFs t FieldHd = pure $ ListType t
-reverseFs t FieldTl = pure $ ListType t
+reverseFs t FieldTl = pure t
reverseFs t FieldFst = fresh >>= \tv -> pure $ TupleType (t, tv)
reverseFs t FieldSnd = fresh >>= \tv -> pure $ TupleType (tv, t)