theorem le_to_or_lt_eq: ∀n,m:nat. n ≤ m → n < m ∨ n = m.
#n #m #lenm (elim lenm) /3/ qed.
+theorem eq_or_gt: ∀n. 0 = n ∨ 0 < n.
+#n elim (le_to_or_lt_eq 0 n ?) // /2 width=1/
+qed-.
+
theorem increasing_to_le2: ∀f:nat → nat. increasing f →
∀m:nat. f 0 ≤ m → ∃i. f i ≤ m ∧ m < f (S i).
#f #incr #m #lem (elim lem)
(∀n. (∀a. f a < n → P a) → ∀a. f a = n → P a) → ∀a. P a.
#A #f #P #H #a
@(f_ind_aux … H) -H [2: // | skip ]
+qed-.
+
+fact f2_ind_aux: ∀A1,A2. ∀f:A1→A2→ℕ. ∀P:relation2 A1 A2.
+ (∀n. (∀a1,a2. f a1 a2 < n → P a1 a2) → ∀a1,a2. f a1 a2 = n → P a1 a2) →
+ ∀n,a1,a2. f a1 a2 = n → P a1 a2.
+#A1 #A2 #f #P #H #n @(nat_elim1 … n) -n #n /3 width=3/ (**) (* auto slow (34s) without #n *)
+qed-.
+
+lemma f2_ind: ∀A1,A2. ∀f:A1→A2→ℕ. ∀P:relation2 A1 A2.
+ (∀n. (∀a1,a2. f a1 a2 < n → P a1 a2) → ∀a1,a2. f a1 a2 = n → P a1 a2) →
+ ∀a1,a2. P a1 a2.
+#A1 #A2 #f #P #H #a1 #a2
+@(f2_ind_aux … H) -H [2: // | skip ]
qed-.
(* More negated equalities **************************************************)