proposition: Lemma der_normalS n G : G^`(n.+1) <| G^`(n).
proof: Proof. by rewrite sub_der1_normal // der_subS. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat fintype) mathcomp(bigop finset binomial fingroup morphism) mathcomp(automorphism quotient gfunctor)