proposition: Lemma is_ballP (A : set R) x : is_ball A -> A x -> `|cpoint A - x| < (radius A)%:num.
proof: Proof. by rewrite /is_ball => /eqP {1}-> /lt_le_trans; exact. 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