proposition: Lemma nonpos_plus_compat x y : x ≤ 0 → y ≤ 0 → x + y ≤ 0.
proof: Proof. intros. rewrite <-(plus_0_r 0). now apply plus_le_compat. Qed.
imports: Coq.setoid_ring.Ring MathClasses.interfaces.abstract_algebra MathClasses.interfaces.orders MathClasses.theory.rings MathClasses.orders.orders MathClasses.orders.maps