proposition: Lemma ball_sym_subproof A : ent A -> [set xy | xy.1 = xy.2] `<=` A.
proof: Proof. rewrite entourageE; move=> [e egt0 sbeA] xy xey. apply: sbeA; rewrite /= xey; exact: ball_center. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed