proposition: Lemma commXXg : [~ x ^+ i, y ^+ j] = [~ x, y] ^+ (i * j).
proof: Proof. by rewrite expgM commgX commXg //; apply: commuteX. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat fintype) mathcomp(bigop finset binomial fingroup morphism) mathcomp(automorphism quotient gfunctor)