proposition: Lemma invg2id x : #[x] = 2 -> x^-1 = x.
proof: Proof. by move=> ox; rewrite invg_expg ox. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)