lemma yle_inv_Y1: ∀n. ∞ ≤ n → n = ∞.
/2 width=3 by yle_inv_Y1_aux/ qed-.
-(* Inversion lemmas on successor ********************************************)
-
-fact yle_inv_succ1_aux: ∀x,y. x ≤ y → ∀m. x = ⫯m → m ≤ ⫰y ∧ ⫯⫰y = y.
-#x #y * -x -y
-[ #x #y #Hxy #m #H elim (ysucc_inv_inj_sn … H) -H
- #n #H1 #H2 destruct elim (le_inv_S1 … Hxy) -Hxy
- #m #Hnm #H destruct /3 width=1 by yle_inj, conj/
-| #x #y #H destruct /2 width=1 by yle_Y, conj/
-]
-qed-.
-
-lemma yle_inv_succ1: ∀m,y. ⫯m ≤ y → m ≤ ⫰y ∧ ⫯⫰y = y.
-/2 width=3 by yle_inv_succ1_aux/ qed-.
-
-lemma yle_inv_succ: ∀m,n. ⫯m ≤ ⫯n → m ≤ n.
-#m #n #H elim (yle_inv_succ1 … H) -H //
+lemma yle_antisym: ∀y,x. x ≤ y → y ≤ x → x = y.
+#x #y #H elim H -x -y
+/4 width=1 by yle_inv_Y1, yle_inv_inj, le_to_le_to_eq, eq_f/
qed-.
(* Basic properties *********************************************************)
#y elim (le_or_ge x y) /3 width=1 by yle_inj, or_introl, or_intror/
qed-.
+(* Inversion lemmas on successor ********************************************)
+
+fact yle_inv_succ1_aux: ∀x,y:ynat. x ≤ y → ∀m. x = ⫯m → m ≤ ⫰y ∧ ⫯⫰y = y.
+#x #y * -x -y
+[ #x #y #Hxy #m #H elim (ysucc_inv_inj_sn … H) -H
+ #n #H1 #H2 destruct elim (le_inv_S1 … Hxy) -Hxy
+ #m #Hnm #H destruct /3 width=1 by yle_inj, conj/
+| #x #y #H destruct /2 width=1 by yle_Y, conj/
+]
+qed-.
+
+lemma yle_inv_succ1: ∀m,y:ynat. ⫯m ≤ y → m ≤ ⫰y ∧ ⫯⫰y = y.
+/2 width=3 by yle_inv_succ1_aux/ qed-.
+
+lemma yle_inv_succ: ∀m,n. ⫯m ≤ ⫯n → m ≤ n.
+#m #n #H elim (yle_inv_succ1 … H) -H //
+qed-.
+
+lemma yle_inv_succ2: ∀x,y. x ≤ ⫯y → ⫰x ≤ y.
+#x #y #Hxy elim (ynat_cases x)
+[ #H destruct //
+| * #m #H destruct /2 width=1 by yle_inv_succ/
+]
+qed-.
+
(* Properties on predecessor ************************************************)
lemma yle_pred_sn: ∀m,n. m ≤ n → ⫰m ≤ n.
lemma yle_refl_SP_dx: ∀x. x ≤ ⫯⫰x.
* // * //
-qed.
+qed.
+
+lemma yle_succ2: ∀x,y. ⫰x ≤ y → x ≤ ⫯y.
+#x #y #Hxy elim (ynat_cases x)
+[ #H destruct //
+| * #m #H destruct /2 width=1 by yle_succ/
+]
+qed-.
(* Main properties **********************************************************)