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