fact: Definition prod_ball x (eps : R) y := ball (fst x) eps (fst y) /\ ball (snd x) eps (snd y).
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed