proposition: Lemma ball_norm_dec x y (e : R) : {ball_norm x e y} + {~ ball_norm x e y}.
proof: Proof. exact: pselect. Qed.
imports: HB(structures) mathcomp(all_ssreflect ssralg ssrint ssrnum finmap matrix) mathcomp(rat interval zmodp vector fieldext falgebra) mathcomp(mathcomp_extra boolp classical_sets functions) mathcomp(cardinality set_interval Rstruct) ereal reals signed topology prodnormedzmodule