proposition: Lemma open_nbhs_entourage (x : U) (A : set (U * U)) : entourage A -> open_nbhs x (to_set A x)^°.
proof: Proof. move=> entA; split; first exact: open_interior. by apply: nbhs_singleton; apply: nbhs_interior; apply: nbhs_entourage. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed