proposition: Lemma quotient_der n G H : G \subset 'N(H) -> G^`(n) / H = (G / H)^`(n).
proof: Proof. exact: morphim_der. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat fintype) mathcomp(bigop finset binomial fingroup morphism) mathcomp(automorphism quotient gfunctor)