proposition: Lemma join_ge_l x y : x <= join x y.
proof: Proof. move: (@set_joinUl (set1 x) (set1 y)); rewrite -ubP; exact. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype choice order) mathcomp(ssrnat bigop classical_sets)