proposition: Lemma commg_normSl G H K : G \subset 'N(H) -> [~: G, H] \subset 'N([~: K, H]).
proof: Proof. by move=> nHG; rewrite normsRr // commg_subr. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat fintype) mathcomp(bigop finset binomial fingroup morphism) mathcomp(automorphism quotient gfunctor)