proposition: Lemma surj_if_exists_right_invC (a b:Set) (f:a->b): (exists s, is_right_inverseC f s) -> surjectiveC f.
proof: Proof. by move=> [s rsf] u; exists (s u); rewrite (right_i_v u rsf). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)