proposition: Lemma groupV x : (x^-1 \in G) = (x \in G).
proof: Proof. by apply/idP/idP; [apply: groupVl | apply: groupVr]. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)