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