proposition: Lemma memV_invg x A : (x^-1 \in A^-1) = (x \in A).
proof: Proof. by rewrite inE invgK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)