nsIn :: Ty -> [TyCon]
tyconsIn (TyF t1 t2)    = tyconsIn t1 `union` tyconsIn t2
tyc
tyconsIn :: Ty -> [TyCon]
tyconsIn (TyF t1 t2)    = tyconsIn t1 `union` tyconsIn t2
tyconsIn (TyF t1 t2)    = tyconsIn t1 `union` tyconsIn t2