proposition: Lemma entourage_invI (E : set (M * M)) : entourage E -> entourage (E `&` E^-1)%classic.
proof: Proof. by move=> ?; apply: filterI; last exact: entourage_inv. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed