proposition: Lemma quotient_subnormG A G : H <| G -> 'N_A(G) / H = 'N_(A / H)(G / H).
proof: Proof. by case/andP=> sHG nHG; rewrite -morphim_subnormG ?ker_coset. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)