intros a; case a; unfold R in |- *; simpl in |- *; auto with arith.
Qed.
(*Is R factorizable ?*)
Notation Factorizable := (factorizable _) (only parsing).

Lemma is_factorizable : factorizable _ R.
unfold factorizable in |- *; unfold R in |- *;
intros M N q q' r r' a a1 a' x x' H1 H2; case a.