proposition: Lemma quotient_set1 x : x \in 'N(H) -> [set x] / H = [set coset H x].
proof: Proof. exact: morphim_set1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)