X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=blobdiff_plain;f=helm%2Fsoftware%2Fmatita%2Fnlibrary%2Farithmetics%2Fnat.ma;h=3152b0dbe3503aeee43bbd0f872b847f87979239;hb=eb4144a401147a44a9620169eb6dafeb8f5a2c17;hp=a1e6fbb230c6f7cd93e444aa96a4fc681bfbacd1;hpb=9f0b17f553522373b51fc4761d5a3e1e9dbbad73;p=helm.git

diff --git a/helm/software/matita/nlibrary/arithmetics/nat.ma b/helm/software/matita/nlibrary/arithmetics/nat.ma
index a1e6fbb23..3152b0dbe 100644
--- a/helm/software/matita/nlibrary/arithmetics/nat.ma
+++ b/helm/software/matita/nlibrary/arithmetics/nat.ma
@@ -36,12 +36,10 @@ ncoercion nat_to_pos: ∀n:nat. n ≠0 →pos ≝ mk_pos on
 *)
 
 ndefinition pred ≝
- λn. match n with
- [ O ⇒  O
- | (S p) ⇒ p].
+ λn. match n with [ O ⇒  O | (S p) ⇒ p].
 
 ntheorem pred_Sn : ∀n. n = pred (S n).
-//. nqed.
+//; nqed.
 
 ntheorem injective_S : injective nat nat S.
 //; nqed.
@@ -55,8 +53,7 @@ ntheorem not_eq_S: ∀n,m:nat. n ≠ m → S n ≠ S m.
 
 ndefinition not_zero: nat → Prop ≝
  λn: nat. match n with
-  [ O ⇒ False
-  | (S p) ⇒ True ].
+  [ O ⇒ False | (S p) ⇒ True ].
 
 ntheorem not_eq_O_S : ∀n:nat. O ≠ S n.
 #n; #eqOS; nchange with (not_zero O); nrewrite > eqOS; //.
@@ -80,10 +77,10 @@ ntheorem nat_elim2 :
 #n0; #Rn0m; #m; ncases m;/2/; nqed.
 
 ntheorem decidable_eq_nat : \forall n,m:nat.decidable (n=m).
-napply nat_elim2;#n;
+napply nat_elim2; #n;
  ##[ ncases n; /2/;
  ##| /3/;
- ##| #m; #Hind; ncases Hind; /3/; (* ??? /2/;#neqnm; /3/; *)
+ ##| #m; #Hind; ncases Hind; /3/;
  ##]
 nqed. 
 
@@ -125,7 +122,7 @@ ntheorem symmetric_plus: symmetric ? plus.
 ntheorem associative_plus : associative nat plus.
 #n; nelim n; nnormalize; //; nqed.
 
-ntheorem assoc_plus1: ∀a,b,c. b + (a + c) = a + (b + c).
+ntheorem assoc_plus1: ∀a,b,c. c + (b + a) = b + c + a.
 //; nqed.
 
 ntheorem injective_plus_r: ∀n:nat.injective nat nat (λm.n+m).
@@ -206,5 +203,593 @@ n = (S(S O))*m \lor n = S ((S(S O))*m).
 nqed.
 *)
 
-(************************** compare ****************************)
+(******************** ordering relations ************************)
+
+ninductive le (n:nat) : nat → Prop ≝
+  | le_n : le n n
+  | le_S : ∀ m:nat. le n m → le n (S m).
+
+interpretation "natural 'less or equal to'" 'leq x y = (le x y).
+
+interpretation "natural 'neither less nor equal to'" 'nleq x y = (Not (le x y)).
+
+ndefinition lt: nat → nat → Prop ≝
+λn,m:nat. S n ≤ m.
+
+interpretation "natural 'less than'" 'lt x y = (lt x y).
+
+interpretation "natural 'not less than'" 'nless x y = (Not (lt x y)).
+
+ndefinition ge: nat \to nat \to Prop \def
+\lambda n,m:nat.m \leq n.
+
+interpretation "natural 'greater or equal to'" 'geq x y = (ge x y).
+
+ndefinition gt: nat \to nat \to Prop \def
+\lambda n,m:nat.m<n.
+
+interpretation "natural 'greater than'" 'gt x y = (gt x y).
+
+interpretation "natural 'not greater than'" 'ngtr x y = (Not (gt x y)).
+
+ntheorem transitive_le : transitive nat le.
+#a; #b; #c; #leab; #lebc;nelim lebc;/2/;
+nqed.
+
+(*
+ntheorem trans_le: \forall n,m,p:nat. n \leq m \to m \leq p \to n \leq p
+\def transitive_le. *)
+
+ntheorem transitive_lt: transitive nat lt.
+#a; #b; #c; #ltab; #ltbc;nelim ltbc;/2/;nqed.
+
+(*
+theorem trans_lt: \forall n,m,p:nat. lt n m \to lt m p \to lt n p
+\def transitive_lt. *)
+
+ntheorem le_S_S: ∀n,m:nat. n ≤ m → S n ≤ S m.
+#n; #m; #lenm; nelim lenm; /2/; nqed.
+
+ntheorem le_O_n : ∀n:nat. O ≤ n.
+#n; nelim n; /2/; nqed.
+
+ntheorem le_n_Sn : ∀n:nat. n ≤ S n.
+/2/; nqed.
+
+ntheorem le_pred_n : ∀n:nat. pred n ≤ n.
+#n; nelim n; //; nqed.
+
+ntheorem monotonic_pred: monotonic ? le pred.
+#n; #m; #lenm; nelim lenm; /2/; nqed.
+
+ntheorem le_S_S_to_le: ∀n,m:nat. S n ≤ S m → n ≤ m.
+/2/; nqed.
+
+ntheorem lt_S_S_to_lt: ∀n,m. S n < S m \to n < m.
+/2/; nqed. 
+
+ntheorem lt_to_lt_S_S: ∀n,m. n < m → S n < S m.
+/2/; nqed.
+
+ntheorem lt_to_not_zero : ∀n,m:nat. n < m → not_zero m.
+#n; #m; #Hlt; nelim Hlt;//; nqed.
+
+(* lt vs. le *)
+ntheorem not_le_Sn_O: ∀ n:nat. S n ≰ O.
+#n; #Hlen0; napply (lt_to_not_zero ?? Hlen0); nqed.
+
+ntheorem not_le_to_not_le_S_S: ∀ n,m:nat. n ≰ m → S n ≰ S m.
+/3/; nqed.
+
+ntheorem not_le_S_S_to_not_le: ∀ n,m:nat. S n ≰ S m → n ≰ m.
+/3/; nqed.
+
+ntheorem decidable_le: ∀n,m. decidable (n≤m).
+napply nat_elim2; #n; /2/;
+#m; #dec; ncases dec;/3/; nqed.
+
+ntheorem decidable_lt: ∀n,m. decidable (n < m).
+#n; #m; napply decidable_le ; nqed.
+
+ntheorem not_le_Sn_n: ∀n:nat. S n ≰ n.
+#n; nelim n; /2/; nqed.
+
+ntheorem lt_S_to_le: ∀n,m:nat. n < S m → n ≤ m.
+/2/; nqed.
+
+ntheorem not_le_to_lt: ∀n,m. n ≰ m → m < n.
+napply nat_elim2; #n;
+ ##[#abs; napply False_ind;/2/;
+ ##|/2/;
+ ##|#m;#Hind;#HnotleSS; napply lt_to_lt_S_S;/3/;
+ ##]
+nqed.
+
+ntheorem lt_to_not_le: ∀n,m. n < m → m ≰ n.
+#n; #m; #Hltnm; nelim Hltnm;/3/; nqed.
+
+ntheorem not_lt_to_le: ∀n,m:nat. n ≮ m → m ≤ n.
+#n; #m; #Hnlt; napply lt_S_to_le;
+(* something strange here: /2/ fails: 
+   we need an extra depths for unfolding not *)
+napply not_le_to_lt; napply Hnlt; nqed. 
+
+ntheorem le_to_not_lt: ∀n,m:nat. n ≤ m → m ≮ n.
+/2/; nqed.
+
+(* lt and le trans *)
+
+ntheorem lt_to_le_to_lt: ∀n,m,p:nat. n < m → m ≤ p → n < p.
+#n; #m; #p; #H; #H1; nelim H1; /2/; nqed.
+
+ntheorem le_to_lt_to_lt: ∀n,m,p:nat. n ≤ m → m < p → n < p.
+#n; #m; #p; #H; nelim H; /3/; nqed.
+
+ntheorem lt_S_to_lt: ∀n,m. S n < m → n < m.
+/2/; nqed.
+
+ntheorem ltn_to_ltO: ∀n,m:nat. n < m → O < m.
+/2/; nqed.
+
+(*
+theorem lt_SO_n_to_lt_O_pred_n: \forall n:nat.
+(S O) \lt n \to O \lt (pred n).
+intros.
+apply (ltn_to_ltO (pred (S O)) (pred n) ?).
+ apply (lt_pred (S O) n);
+ [ apply (lt_O_S O) 
+ | assumption
+ ]
+qed. *)
+
+ntheorem lt_O_n_elim: ∀n:nat. O < n → 
+  ∀P:nat → Prop.(∀m:nat.P (S m)) → P n.
+#n; nelim n; //; #abs; napply False_ind; /2/; nqed.
+
+(*
+theorem lt_pred: \forall n,m. 
+  O < n \to n < m \to pred n < pred m. 
+apply nat_elim2
+  [intros.apply False_ind.apply (not_le_Sn_O ? H)
+  |intros.apply False_ind.apply (not_le_Sn_O ? H1)
+  |intros.simplify.unfold.apply le_S_S_to_le.assumption
+  ]
+qed.
+
+theorem S_pred: \forall n:nat.lt O n \to eq nat n (S (pred n)).
+intro.elim n.apply False_ind.exact (not_le_Sn_O O H).
+apply eq_f.apply pred_Sn.
+qed.
+
+theorem le_pred_to_le:
+ ∀n,m. O < m → pred n ≤ pred m → n ≤ m.
+intros 2;
+elim n;
+[ apply le_O_n
+| simplify in H2;
+  rewrite > (S_pred m);
+  [ apply le_S_S;
+    assumption
+  | assumption
+  ]
+].
+qed.
+
+*)
+
+(* le to lt or eq *)
+ntheorem le_to_or_lt_eq: ∀n,m:nat. n ≤ m → n < m ∨ n = m.
+#n; #m; #lenm; nelim lenm; /3/; nqed.
+
+(* not eq *)
+ntheorem lt_to_not_eq : ∀n,m:nat. n < m → n ≠ m.
+/2/; nqed.
+
+(*not lt 
+ntheorem eq_to_not_lt: ∀a,b:nat. a = b → a ≮ b.
+intros.
+unfold Not.
+intros.
+rewrite > H in H1.
+apply (lt_to_not_eq b b)
+[ assumption
+| reflexivity
+]
+qed. 
+
+theorem lt_n_m_to_not_lt_m_Sn: ∀n,m. n < m → m ≮ S n.
+intros;
+unfold Not;
+intro;
+unfold lt in H;
+unfold lt in H1;
+generalize in match (le_S_S ? ? H);
+intro;
+generalize in match (transitive_le ? ? ? H2 H1);
+intro;
+apply (not_le_Sn_n ? H3).
+qed. *)
+
+ntheorem not_eq_to_le_to_lt: ∀n,m. n≠m → n≤m → n<m.
+#n; #m; #Hneq; #Hle; ncases (le_to_or_lt_eq ?? Hle); //;
+#Heq; nelim (Hneq Heq); nqed.
+
+(* le elimination *)
+ntheorem le_n_O_to_eq : ∀n:nat. n ≤ O → O=n.
+#n; ncases n; //; #a ; #abs; nelim (not_le_Sn_O ? abs); nqed.
+
+ntheorem le_n_O_elim: ∀n:nat. n ≤ O → ∀P: nat →Prop. P O → P n.
+#n; ncases n; //; #a; #abs; nelim (not_le_Sn_O ? abs); nqed. 
+
+ntheorem le_n_Sm_elim : ∀n,m:nat.n ≤ S m → 
+∀P:Prop. (S n ≤ S m → P) → (n=S m → P) → P.
+#n; #m; #Hle; #P; nelim Hle; /3/; nqed.
+
+(* le and eq *)
+
+ntheorem le_to_le_to_eq: ∀n,m. n ≤ m → m ≤ n → n = m.
+napply nat_elim2; /3/; nqed.
+
+ntheorem lt_O_S : \forall n:nat. O < S n.
+/2/; nqed.
+
+(*
+(* other abstract properties *)
+theorem antisymmetric_le : antisymmetric nat le.
+unfold antisymmetric.intros 2.
+apply (nat_elim2 (\lambda n,m.(n \leq m \to m \leq n \to n=m))).
+intros.apply le_n_O_to_eq.assumption.
+intros.apply False_ind.apply (not_le_Sn_O ? H).
+intros.apply eq_f.apply H.
+apply le_S_S_to_le.assumption.
+apply le_S_S_to_le.assumption.
+qed.
+
+theorem antisym_le: \forall n,m:nat. n \leq m \to m \leq n \to n=m
+\def antisymmetric_le.
+
+theorem le_n_m_to_lt_m_Sn_to_eq_n_m: ∀n,m. n ≤ m → m < S n → n=m.
+intros;
+unfold lt in H1;
+generalize in match (le_S_S_to_le ? ? H1);
+intro;
+apply antisym_le;
+assumption.
+qed.
+*)
+
+(* well founded induction principles *)
+
+ntheorem nat_elim1 : ∀n:nat.∀P:nat → Prop. 
+(∀m.(∀p. p < m → P p) → P m) → P n.
+#n; #P; #H; 
+ncut (∀q:nat. q ≤ n → P q);/2/;
+nelim n; 
+ ##[#q; #HleO; (* applica male *) 
+    napply (le_n_O_elim ? HleO);
+    napply H; #p; #ltpO;
+    napply False_ind; /2/; 
+ ##|#p; #Hind; #q; #HleS; 
+    napply H; #a; #lta; napply Hind;
+    napply le_S_S_to_le;/2/;
+ ##]
+nqed.
+
+(* some properties of functions *)
+(*
+definition increasing \def \lambda f:nat \to nat. 
+\forall n:nat. f n < f (S n).
+
+theorem increasing_to_monotonic: \forall f:nat \to nat.
+increasing f \to monotonic nat lt f.
+unfold monotonic.unfold lt.unfold increasing.unfold lt.intros.elim H1.apply H.
+apply (trans_le ? (f n1)).
+assumption.apply (trans_le ? (S (f n1))).
+apply le_n_Sn.
+apply H.
+qed.
+
+theorem le_n_fn: \forall f:nat \to nat. (increasing f) 
+\to \forall n:nat. n \le (f n).
+intros.elim n.
+apply le_O_n.
+apply (trans_le ? (S (f n1))).
+apply le_S_S.apply H1.
+simplify in H. unfold increasing in H.unfold lt in H.apply H.
+qed.
+
+theorem increasing_to_le: \forall f:nat \to nat. (increasing f) 
+\to \forall m:nat. \exists i. m \le (f i).
+intros.elim m.
+apply (ex_intro ? ? O).apply le_O_n.
+elim H1.
+apply (ex_intro ? ? (S a)).
+apply (trans_le ? (S (f a))).
+apply le_S_S.assumption.
+simplify in H.unfold increasing in H.unfold lt in H.
+apply H.
+qed.
+
+theorem increasing_to_le2: \forall f:nat \to nat. (increasing f) 
+\to \forall m:nat. (f O) \le m \to 
+\exists i. (f i) \le m \land m <(f (S i)).
+intros.elim H1.
+apply (ex_intro ? ? O).
+split.apply le_n.apply H.
+elim H3.elim H4.
+cut ((S n1) < (f (S a)) \lor (S n1) = (f (S a))).
+elim Hcut.
+apply (ex_intro ? ? a).
+split.apply le_S. assumption.assumption.
+apply (ex_intro ? ? (S a)).
+split.rewrite < H7.apply le_n.
+rewrite > H7.
+apply H.
+apply le_to_or_lt_eq.apply H6.
+qed.
+*)
+
+(*********************** monotonicity ***************************)
+ntheorem monotonic_le_plus_r: 
+∀n:nat.monotonic nat le (λm.n + m).
+#n; #a; #b; nelim n; nnormalize; //;
+#m; #H; #leab;napply le_S_S; /2/; nqed.
+
+(*
+ntheorem le_plus_r: ∀p,n,m:nat. n ≤ m → p + n ≤ p + m
+≝ monotonic_le_plus_r. *)
+
+ntheorem monotonic_le_plus_l: 
+∀m:nat.monotonic nat le (λn.n + m).
+/2/; nqed.
+
+(*
+ntheorem le_plus_l: \forall p,n,m:nat. n \le m \to n + p \le m + p
+\def monotonic_le_plus_l. *)
+
+ntheorem le_plus: ∀n1,n2,m1,m2:nat. n1 ≤ n2  \to m1 ≤ m2 
+→ n1 + m1 ≤ n2 + m2.
+#n1; #n2; #m1; #m2; #len; #lem; napply transitive_le;
+/2/; nqed.
+
+ntheorem le_plus_n :∀n,m:nat. m ≤ n + m.
+/2/; nqed. 
+
+ntheorem le_plus_n_r :∀n,m:nat. m ≤ m + n.
+/2/; nqed.
+
+ntheorem eq_plus_to_le: ∀n,m,p:nat.n=m+p → m ≤ n.
+//; nqed.
+
+ntheorem le_plus_to_le: ∀a,n,m. a + n ≤ a + m → n ≤ m.
+#a; nelim a; /3/; nqed. 
+
+ntheorem le_plus_to_le_r: ∀a,n,m. n + a ≤ m +a → n ≤ m.
+/2/; nqed. 
 
+(* times *)
+ntheorem monotonic_le_times_r: 
+∀n:nat.monotonic nat le (λm. n * m).
+#n; #x; #y; #lexy; nelim n; nnormalize;//;(* lento /2/;*)
+#a; #lea; napply le_plus; //;
+nqed.
+
+(*
+ntheorem le_times_r: \forall p,n,m:nat. n \le m \to p*n \le p*m
+\def monotonic_le_times_r. *)
+
+ntheorem monotonic_le_times_l: 
+∀m:nat.monotonic nat le (λn.n*m).
+/2/; nqed.
+
+(*
+theorem le_times_l: \forall p,n,m:nat. n \le m \to n*p \le m*p
+\def monotonic_le_times_l. *)
+
+ntheorem le_times: ∀n1,n2,m1,m2:nat. 
+n1 ≤ n2  → m1 ≤ m2 → n1*m1 ≤ n2*m2.
+#n1; #n2; #m1; #m2; #len; #lem; 
+napply transitive_le; (* /2/ slow *)
+ ##[ ##| napply monotonic_le_times_l;//; 
+     ##| napply monotonic_le_times_r;//;
+ ##]
+nqed.
+
+ntheorem lt_times_n: ∀n,m:nat. O < n → m ≤ n*m.
+(* bello *)
+/2/; nqed.
+
+ntheorem le_times_to_le: 
+∀a,n,m. O < a → a * n ≤ a * m → n ≤ m.
+#a; napply nat_elim2; nnormalize;
+  ##[//;
+  ##|#n; #H1; #H2; napply False_ind;
+     ngeneralize in match H2;
+     napply lt_to_not_le;
+     napply (transitive_le ? (S n));/2/;
+  ##|#n; #m; #H; #lta; #le;
+     napply le_S_S; napply H; /2/;
+  ##]
+nqed.
+
+ntheorem le_S_times_2: ∀n,m.O < m → n ≤ m → n < 2*m.
+#n; #m; #posm; #lenm; (* interessante *)
+nnormalize; napplyS (le_plus n); //; nqed.
+
+(************************** minus ******************************)
+
+nlet rec minus n m ≝ 
+ match n with 
+ [ O ⇒ O
+ | S p ⇒ 
+	match m with
+	  [ O ⇒ S p
+    | S q ⇒ minus p q ]].
+        
+interpretation "natural minus" 'minus x y = (minus x y).
+
+ntheorem minus_S_S: ∀n,m:nat.S n - S m = n -m.
+//; nqed.
+
+ntheorem minus_O_n: ∀n:nat.O=O-n.
+#n; ncases n; //; nqed.
+
+ntheorem minus_n_O: ∀n:nat.n=n-O.
+#n; ncases n; //; nqed.
+
+ntheorem minus_n_n: ∀n:nat.O=n-n.
+#n; nelim n; //; nqed.
+
+ntheorem minus_Sn_n: ∀n:nat. S O = (S n)-n.
+#n; nelim n; //; nqed.
+
+ntheorem minus_Sn_m: ∀m,n:nat. m ≤ n → S n -m = S (n-m).
+(* qualcosa da capire qui 
+#n; #m; #lenm; nelim lenm; napplyS refl_eq. *)
+napply nat_elim2; 
+  ##[//
+  ##|#n; #abs; napply False_ind;/2/;
+  ##|/3/;
+  ##]
+nqed.
+
+ntheorem eq_minus_S_pred: ∀n,m. n - (S m) = pred(n -m).
+napply nat_elim2; //; nqed.
+
+ntheorem plus_minus:
+∀m,n,p:nat. m ≤ n → (n-m)+p = (n+p)-m.
+napply nat_elim2; 
+  ##[//
+  ##|#n; #p; #abs; napply False_ind;/2/;
+  ##|nnormalize;/3/;
+  ##]
+nqed.
+
+ntheorem minus_plus_m_m: ∀n,m:nat.n = (n+m)-m.
+#n; #m; napplyS (plus_minus m m n); //; nqed.
+
+ntheorem plus_minus_m_m: ∀n,m:nat.
+m \leq n \to n = (n-m)+m.
+#n; #m; #lemn; napplyS symmetric_eq; 
+napplyS (plus_minus m n m); //; nqed.
+
+ntheorem le_plus_minus_m_m: ∀n,m:nat. n ≤ (n-m)+m.
+#n; nelim n;
+  ##[//
+  ##|#a; #Hind; #m; ncases m;/2/;  
+  ##]
+nqed.
+
+ntheorem minus_to_plus :∀n,m,p:nat.
+  m ≤ n → n-m = p → n = m+p.
+#n; #m; #p; #lemn; #eqp; napplyS plus_minus_m_m; //;
+nqed.
+
+ntheorem plus_to_minus :∀n,m,p:nat.n = m+p → n-m = p.
+(* /4/ done in 43.5 *)
+#n; #m; #p; #eqp; 
+napply symmetric_eq;
+napplyS (minus_plus_m_m p m);
+nqed.
+
+ntheorem minus_pred_pred : ∀n,m:nat. O < n → O < m → 
+pred n - pred m = n - m.
+#n; #m; #posn; #posm;
+napply (lt_O_n_elim n posn);
+napply (lt_O_n_elim m posm);//.
+nqed.
+
+(*
+theorem eq_minus_n_m_O: \forall n,m:nat.
+n \leq m \to n-m = O.
+intros 2.
+apply (nat_elim2 (\lambda n,m.n \leq m \to n-m = O)).
+intros.simplify.reflexivity.
+intros.apply False_ind.
+apply not_le_Sn_O;
+[2: apply H | skip].
+intros.
+simplify.apply H.apply le_S_S_to_le. apply H1.
+qed.
+
+theorem le_SO_minus: \forall n,m:nat.S n \leq m \to S O \leq m-n.
+intros.elim H.elim (minus_Sn_n n).apply le_n.
+rewrite > minus_Sn_m.
+apply le_S.assumption.
+apply lt_to_le.assumption.
+qed.
+
+theorem minus_le_S_minus_S: \forall n,m:nat. m-n \leq S (m-(S n)).
+intros.
+apply (nat_elim2 (\lambda n,m.m-n \leq S (m-(S n)))).
+intro.elim n1.simplify.apply le_n_Sn.
+simplify.rewrite < minus_n_O.apply le_n.
+intros.simplify.apply le_n_Sn.
+intros.simplify.apply H.
+qed.
+
+theorem lt_minus_S_n_to_le_minus_n : \forall n,m,p:nat. m-(S n) < p \to m-n \leq p. 
+intros 3.intro.
+(* autobatch *)
+(* end auto($Revision: 9739 $) proof: TIME=1.33 SIZE=100 DEPTH=100 *)
+apply (trans_le (m-n) (S (m-(S n))) p).
+apply minus_le_S_minus_S.
+assumption.
+qed.
+
+theorem le_minus_m: \forall n,m:nat. n-m \leq n.
+intros.apply (nat_elim2 (\lambda m,n. n-m \leq n)).
+intros.rewrite < minus_n_O.apply le_n.
+intros.simplify.apply le_n.
+intros.simplify.apply le_S.assumption.
+qed.
+
+theorem lt_minus_m: \forall n,m:nat. O < n \to O < m \to n-m \lt n.
+intros.apply (lt_O_n_elim n H).intro.
+apply (lt_O_n_elim m H1).intro.
+simplify.unfold lt.apply le_S_S.apply le_minus_m.
+qed.
+
+theorem minus_le_O_to_le: \forall n,m:nat. n-m \leq O \to n \leq m.
+intros 2.
+apply (nat_elim2 (\lambda n,m:nat.n-m \leq O \to n \leq m)).
+intros.apply le_O_n.
+simplify.intros. assumption.
+simplify.intros.apply le_S_S.apply H.assumption.
+qed.
+*)
+
+(* monotonicity and galois *)
+
+ntheorem monotonic_le_minus_l: 
+∀p,q,n:nat. q ≤ p → q-n ≤ p-n.
+napply nat_elim2; #p; #q;
+  ##[#lePO; napply (le_n_O_elim ? lePO);//;
+  ##|//;
+  ##|#Hind; #n; ncases n;
+    ##[//;
+    ##|#a; #leSS; napply Hind; /2/;
+    ##]
+  ##]
+nqed.
+
+ntheorem le_minus_to_plus: ∀n,m,p. n-m ≤ p → n≤ p+m.
+#n; #m; #p; #lep;
+napply transitive_le;
+  ##[##|napply le_plus_minus_m_m
+  ##|napply monotonic_le_plus_l;//;
+  ##]
+nqed.
+
+ntheorem le_plus_to_minus: ∀n,m,p. n ≤ p+m → n-m ≤ p.
+#n; #m; #p; #lep;
+(* bello *)
+napplyS monotonic_le_minus_l;//;
+nqed.
+
+ntheorem monotonic_le_minus_r: 
+∀p,q,n:nat. q ≤ p → n-p ≤ n-q.
+#p; #q; #n; #lepq;
+napply le_plus_to_minus;
+napply (transitive_le ??? (le_plus_minus_m_m ? q));/2/;
+nqed.