proposition: Lemma fct_ball_sym (x y : T -> U) (e : R) : fct_ball x e y -> fct_ball y e x.
proof: Proof. by move=> P t; 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