fact: Definition subspace_ent := filter_from (@entourage X) (fun E => [set xy | (xy.1 = xy.2) \/ (A xy.1 /\ A xy.2 /\ E xy)]).
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed