proposition: Lemma is_ball_ball x r : is_ball (ball x r).
proof: Proof. have [r0|/ball0 ->] := ltP 0 r; last exact: is_ball0. by apply/eqP; rewrite cpoint_ball// (radius_ball _ (ltW r0)). 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