proposition: Lemma commuteV x y : commute x y -> commute x y^-1.
proof: Proof. by move=> cxy; apply: (@mulIg y); rewrite mulgKV -mulgA cxy mulKg. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)