proposition: Lemma join_sl_le_spec x y : x ≤ y ↔ x ⊔ y = y.
proof: Proof. split; intros E. now apply join_r. rewrite <-E. now apply join_ub_l. Qed.
imports: MathClasses.interfaces.abstract_algebra MathClasses.interfaces.orders MathClasses.orders.maps MathClasses.theory.lattices