proposition: Lemma fsetUSS A B C D : A `<=` C -> B `<=` D -> A `|` B `<=` C `|` D.
proof: Proof. by move=> /(fsetSU B) /fsubset_trans sAC /(fsetUS C)/sAC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)