proposition: Lemma card_le1_trivg : #|G| <= 1 -> G :=: 1.
proof: Proof. by rewrite -trivg_card_le1; move/eqP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)