proposition: Lemma RhullK : {in (@is_interval _ : set (set R)), cancel (@Rhull R) pred_set}.
proof: Proof. by move=> X /asboolP iX; apply/esym/is_intervalP. Qed.
imports: mathcomp(all_ssreflect ssralg ssrnum ssrint interval) mathcomp(finmap fingroup perm rat) mathcomp(mathcomp_extra boolp classical_sets functions) HB(structures) reals ereal signed topology normedtype sequences