proposition: Lemma ker_quotm : 'ker quotm = 'ker f / H.
proof: Proof. by rewrite -morphpre_quotm /quotient morphim1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)