fact: Definition weak_ent : set_system (S * S) := filter_from (@entourage U) (fun V => (map_pair f)@^-1` V).
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed