fact: Definition prod_ent := [set A : set ((U * V) * (U * V)) | filter_prod (@entourage U) (@entourage V) [set ((xy.1.1,xy.2.1),(xy.1.2,xy.2.2)) | xy in A]].
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed