fact: Definition ball_ (R : numDomainType) (V : zmodType) (norm : V -> R) (x : V) (e : R) := [set y | norm (x - y) < e].
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed