proposition: Lemma commgCV x y : x * y = [~ x^-1, y^-1] * (y * x).
proof: Proof. by rewrite commgEl !mulgA !invgK !mulgKV. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)