proposition: Lemma invg_expg x : x^-1 = x ^+ #[x].-1.
proof: Proof. by apply/eqP; rewrite eq_invg_mul -expgS prednK ?expg_order. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)