fact: Definition semi_setD_closed := forall A B, G A -> G B -> exists D, [/\ finite_set D, D `<=` G, A `\` B = \bigcup_(X in D) X & trivIset D id].
imports: mathcomp(all_ssreflect all_algebra finmap) mathcomp(mathcomp_extra boolp classical_sets functions) mathcomp(cardinality fsbigop ) reals ereal signed topology normedtype sequences esum numfun HB(structures)