proposition: Lemma weak_ballE (e : R) (x : S) : f@^-1` (ball (f x) e) = ball x e.
proof: Proof. by []. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed