proposition: Lemma dergS n G H : G \subset H -> G^`(n) \subset H^`(n).
proof: Proof. by move=> sGH; elim: n => // n IHn; apply: commgSS. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat fintype) mathcomp(bigop finset binomial fingroup morphism) mathcomp(automorphism quotient gfunctor)