proposition: Lemma invMg x y : (x * y)^-1 = y^-1 * x^-1.
proof: Proof. exact: invMg_subproof. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)