proposition: Lemma ball_triangle_subproof A : ent A -> ent (A^-1)%classic.
proof: Proof. rewrite entourageE => - [e egt0 sbeA]. by exists e => // xy xye; apply: sbeA; apply: ball_sym. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed