proposition: Lemma can2_rmorphism f' : cancel f f' -> cancel f' f -> multiplicative f'.
proof: Proof. move=> fK f'K. by split=> [x y|]; apply: (canLR fK); rewrite /= (rmorphM, rmorph1) ?f'K. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)