proposition: Lemma quotient_class x A : x \in 'N(H) -> A \subset 'N(H) -> x ^: A / H = coset H x ^: (A / H).
proof: Proof. exact: morphim_class. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)