(* *)
(**************************************************************************)
-include "ground/arith/nat_pred_succ.ma".
+include "ground/arith/nat_le_pred.ma".
include "ground/arith/nat_lt.ma".
(* STRICT ORDER FOR NON-NEGATIVE INTEGERS ***********************************)
-(* Constructions with npred *************************************************)
+(* Destructions with npred **************************************************)
-lemma nlt_zero_sn (m): m = โโm โ ๐ < m.
-// qed.
+(*** S_pred lt_succ_pred lt_inv_O1 *)
+lemma nlt_des_gen (m) (n): m < n โ n = โโn.
+#m #n @(nat_ind_succ โฆ n) -n //
+#H elim (nlt_inv_zero_dx โฆ H)
+qed-.
(* Inversions with npred ****************************************************)
-(*** S_pred *)
-lemma nlt_inv_zero_sn (m): ๐ < m โ m = โโm.
-#m @(nat_ind โฆ m) -m //
-#H elim (nlt_inv_refl โฆ H)
+(*** lt_inv_gen *)
+lemma nlt_inv_gen (m) (n): m < n โ โงโง m โค โn & n = โโn.
+/2 width=1 by nle_inv_succ_sn/ qed-.
+
+(*** lt_inv_S1 *)
+lemma nlt_inv_succ_sn (m) (n): โm < n โ โงโง m < โn & n = โโn.
+/2 width=1 by nle_inv_succ_sn/ qed-.
+
+lemma nlt_inv_pred_dx (m) (n): m < โn โ โm < n.
+#m #n #H >(nlt_des_gen (๐) n)
+[ /2 width=1 by nlt_succ_bi/
+| /3 width=3 by nle_nlt_trans, nlt_nle_trans/
+]
+qed-.
+
+lemma nlt_inv_pred_bi (m) (n):
+ โm < โn โ m < n.
+/3 width=3 by nlt_inv_pred_dx, nle_nlt_trans/
qed-.
+
+(* Constructions with npred *************************************************)
+
+lemma nlt_zero_sn (n): n = โโn โ ๐ < n.
+// qed.
+
+(*** monotonic_lt_pred *)
+lemma nlt_pred_bi (m) (n): ๐ < m โ m < n โ โm < โn.
+#m #n #Hm #Hmn
+@nle_inv_succ_bi
+<(nlt_des_gen โฆ Hm) -Hm
+<(nlt_des_gen โฆ Hmn) //
+qed.