proposition: Lemma quotW P : (forall y : T, P (\pi_qT y)) -> forall x : qT, P x.
proof: Proof. by move=> Py x; rewrite -[x]reprK; apply: Py. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice) mathcomp(seq fintype)