extract_ty (MonoFunTy ty1 ty2) acc = extract_ty ty1 (extract_ty ty2 acc)
extract_ty (MonoDictTy cls tys) acc = foldr extract_ty (cls : acc) tys
extract_ty (MonoUsgTy usg ty) acc = extract_ty ty acc
+extract_ty (MonoUsgForAllTy uv ty) acc = extract_ty ty acc
extract_ty (MonoTyVar tv) acc = tv : acc
extract_ty (HsForAllTy Nothing ctxt ty) acc = extract_ctxt ctxt (extract_ty ty acc)
extract_ty (HsForAllTy (Just tvs) ctxt ty)