proposition: Lemma join_sl_le_spec x y : x ≤ y <-> x ⊔ y = y.
proof: Proof. split; intros E. - apply join_r. trivial. - rewrite <-E. apply join_ub_l. Qed.
imports: HoTT.Classes.interfaces.abstract_algebra HoTT.Classes.interfaces.orders HoTT.Classes.orders.semirings HoTT.Classes.theory.lattices