extenv bs1 (TEnv bs2 vs) = TEnv (E.extenv bs1 bs2) (vs `unionS` fromListS (tv (map snd bs1)))
extenv bs1 (TEnv bs2 vs) = TEnv (E.extenv bs1 bs2) (vs `unionS` fromListS (tv (map snd bs1)))
extenv bs1 (TEnv bs2 vs) = TEnv (E.extenv bs1 bs2) (vs `unionS` fromListS (tv (map snd bs1)))