theorem le_plus: ∀n1,n2,m1,m2:nat. n1 ≤ n2 → m1 ≤ m2
→ n1 + m1 ≤ n2 + m2.
#n1 #n2 #m1 #m2 #len #lem @(transitive_le ? (n1+m2))
-/2/ qed-.
+/2/ qed.
theorem le_plus_n :∀n,m:nat. m ≤ n + m.
/2/ qed.
@(ex_intro ?? (S a)) /2/
qed.
+(* thus is le_plus
lemma le_plus_compatible: ∀x1,x2,y1,y2. x1 ≤ y1 → x2 ≤ y2 → x1 + x2 ≤ y1 + y2.
-/3 by le_minus_to_plus, monotonic_le_plus_r, transitive_le/ qed.
+#x1 #y1 #x2 #y2 #H1 #H2 /2/ @le_plus // /2/ /3 by le_minus_to_plus, monotonic_le_plus_r, transitive_le/ qed.
+*)
(* lt *)