proposition: Lemma entourage_inv (A : set (M * M)) : entourage A -> entourage (A^-1)%classic.
proof: Proof. exact: entourage_inv_subproof. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed