proposition: Lemma fsubUsetP A B C : reflect (A `<=` C /\ B `<=` C) (A `|` B `<=` C).
proof: Proof. by rewrite fsubUset; exact: andP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)