X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=blobdiff_plain;f=matita%2Fmatita%2Flib%2Farithmetics%2Fnat.ma;h=249bd274e74622e59d69ab7d77782a86c713bc4d;hb=80178d6cf86b78bb9fc47f397f4bcfb1fd15a24f;hp=a20311ced3047dd62ab4efb4c4f106a912a2ad7c;hpb=aacce10080ef24f9851d760294c3e5d8233440cc;p=helm.git

diff --git a/matita/matita/lib/arithmetics/nat.ma b/matita/matita/lib/arithmetics/nat.ma
index a20311ced..249bd274e 100644
--- a/matita/matita/lib/arithmetics/nat.ma
+++ b/matita/matita/lib/arithmetics/nat.ma
@@ -1,4 +1,4 @@
-  (*
+(*
     ||M||  This file is part of HELM, an Hypertextual, Electronic        
     ||A||  Library of Mathematics, developed at the Computer Science     
     ||T||  Department of the University of Bologna, Italy.                     
@@ -11,6 +11,10 @@
 
 include "basics/relations.ma".
 
+(* Definitions **************************************************************)
+
+(* natural numbers *)
+
 inductive nat : Type[0] ≝
   | O : nat
   | S : nat → nat.
@@ -22,27 +26,62 @@ alias num (instance 0) = "natural number".
 definition pred ≝
  λn. match n with [ O ⇒ O | S p ⇒ p].
 
-theorem pred_Sn : ∀n. n = pred (S n).
-// qed.
+definition not_zero: nat → Prop ≝
+ λn: nat. match n with [ O ⇒ False | (S p) ⇒ True ].
 
-theorem injective_S : injective nat nat S.
-// qed.
+(* order relations *)
 
-(*
-theorem inj_S : \forall n,m:nat.(S n)=(S m) \to n=m.
-//. qed. *)
+inductive le (n:nat) : nat → Prop ≝
+  | le_n : le n n
+  | le_S : ∀ m:nat. le n m → le n (S m).
 
-theorem not_eq_S: ∀n,m:nat. n ≠ m → S n ≠ S m.
-/2/ qed.
+interpretation "natural 'less or equal to'" 'leq x y = (le x y).
 
-definition not_zero: nat → Prop ≝
- λn: nat. match n with [ O ⇒ False | (S p) ⇒ True ].
+interpretation "natural 'neither less nor equal to'" 'nleq x y = (Not (le x y)).
 
-theorem not_eq_O_S : ∀n:nat. O ≠ S n.
-#n @nmk #eqOS (change with (not_zero O)) >eqOS // qed.
+definition lt: nat → nat → Prop ≝ λn,m. S n ≤ m.
 
-theorem not_eq_n_Sn: ∀n:nat. n ≠ S n.
-#n (elim n) /2/ qed.
+interpretation "natural 'less than'" 'lt x y = (lt x y).
+
+interpretation "natural 'not less than'" 'nless x y = (Not (lt x y)).
+
+definition ge: nat → nat → Prop ≝ λn,m.m ≤ n.
+
+interpretation "natural 'greater or equal to'" 'geq x y = (ge x y).
+
+definition gt: nat → nat → Prop ≝ λn,m.m<n.
+
+interpretation "natural 'greater than'" 'gt x y = (gt x y).
+
+interpretation "natural 'not greater than'" 'ngtr x y = (Not (gt x y)).
+
+(* abstract properties *)
+
+definition increasing ≝ λf:nat → nat. ∀n:nat. f n < f (S n).
+
+(* arithmetic operations *)
+
+let rec plus n m ≝ 
+ match n with [ O ⇒ m | S p ⇒ S (plus p m) ].
+
+interpretation "natural plus" 'plus x y = (plus x y).
+
+let rec times n m ≝ 
+ match n with [ O ⇒ 0 | S p ⇒ m + (times p m) ].
+
+interpretation "natural times" 'times x y = (times x y).
+
+let 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).
+
+(* Generic conclusion ******************************************************)
 
 theorem nat_case:
  ∀n:nat.∀P:nat → Prop. 
@@ -57,41 +96,30 @@ theorem nat_elim2 :
   → ∀n,m:nat. R n m.
 #R #ROn #RSO #RSS #n (elim n) // #n0 #Rn0m #m (cases m) /2/ qed.
 
-theorem decidable_eq_nat : ∀n,m:nat.decidable (n=m).
-@nat_elim2 #n [ (cases n) /2/ | /3/ | #m #Hind (cases Hind) /3/]
-qed. 
-
-(*************************** plus ******************************)
+lemma le_gen: ∀P:nat → Prop.∀n.(∀i. i ≤ n → P i) → P n.
+/2/ qed.
 
-let rec plus n m ≝ 
- match n with [ O ⇒ m | S p ⇒ S (plus p m) ].
+(* Equalities ***************************************************************)
 
-interpretation "natural plus" 'plus x y = (plus x y).
+theorem pred_Sn : ∀n. n = pred (S n).
+// qed.
 
-theorem plus_O_n: ∀n:nat. n = O+n.
+theorem injective_S : injective nat nat S.
 // qed.
 
-(*
-theorem plus_Sn_m: ∀n,m:nat. S (n + m) = S n + m.
+theorem S_pred: ∀n. 0 < n → S(pred n) = n.
+#n #posn (cases posn) //
+qed.
+
+theorem plus_O_n: ∀n:nat. n = 0 + n.
 // qed.
-*)
 
-theorem plus_n_O: ∀n:nat. n = n+O.
+theorem plus_n_O: ∀n:nat. n = n + 0.
 #n (elim n) normalize // qed.
 
 theorem plus_n_Sm : ∀n,m:nat. S (n+m) = n + S m.
 #n (elim n) normalize // qed.
 
-(*
-theorem plus_Sn_m1: ∀n,m:nat. S m + n = n + S m.
-#n (elim n) normalize // qed.
-*)
-
-(* deleterio?
-theorem plus_n_1 : ∀n:nat. S n = n+1.
-// qed.
-*)
-
 theorem commutative_plus: commutative ? plus.
 #n (elim n) normalize // qed. 
 
@@ -104,29 +132,16 @@ theorem assoc_plus1: ∀a,b,c. c + (b + a) = b + c + a.
 theorem injective_plus_r: ∀n:nat.injective nat nat (λm.n+m).
 #n (elim n) normalize /3/ qed.
 
-(* theorem inj_plus_r: \forall p,n,m:nat. p+n = p+m \to n=m
-\def injective_plus_r. 
-
-theorem injective_plus_l: ∀m:nat.injective nat nat (λn.n+m).
-/2/ qed. *)
-
-(* theorem inj_plus_l: \forall p,n,m:nat. n+p = m+p \to n=m
-\def injective_plus_l. *)
-
-(*************************** times *****************************)
-
-let rec times n m ≝ 
- match n with [ O ⇒ O | S p ⇒ m+(times p m) ].
-
-interpretation "natural times" 'times x y = (times x y).
+theorem injective_plus_l: ∀n:nat.injective nat nat (λm.m+n). 
+/2/ qed.
 
 theorem times_Sn_m: ∀n,m:nat. m+n*m = S n*m.
 // qed.
 
-theorem times_O_n: ∀n:nat. O = O*n.
+theorem times_O_n: ∀n:nat. 0 = 0 * n.
 // qed.
 
-theorem times_n_O: ∀n:nat. O = n*O.
+theorem times_n_O: ∀n:nat. 0 = n * 0.
 #n (elim n) // qed.
 
 theorem times_n_Sm : ∀n,m:nat. n+(n*m) = n*(S m).
@@ -135,9 +150,6 @@ theorem times_n_Sm : ∀n,m:nat. n+(n*m) = n*(S m).
 theorem commutative_times : commutative nat times. 
 #n (elim n) normalize // qed. 
 
-(* variant sym_times : \forall n,m:nat. n*m = m*n \def
-symmetric_times. *)
-
 theorem distributive_times_plus : distributive nat times plus.
 #n (elim n) normalize // qed.
 
@@ -152,85 +164,69 @@ lemma times_times: ∀x,y,z. x*(y*z) = y*(x*z).
 // qed. 
 
 theorem times_n_1 : ∀n:nat. n = n * 1.
-#n // qed.
-
-(* ci servono questi risultati? 
-theorem times_O_to_O: ∀n,m:nat.n*m=O → n=O ∨ m=O.
-napply nat_elim2 /2/ 
-#n #m #H normalize #H1 napply False_ind napply not_eq_O_S
 // qed.
-  
-theorem times_n_SO : ∀n:nat. n = n * S O.
-#n // qed.
-
-theorem times_SSO_n : ∀n:nat. n + n = (S(S O)) * n.
-normalize // qed.
 
-nlemma times_SSO: \forall n.(S(S O))*(S n) = S(S((S(S O))*n)).
+theorem minus_S_S: ∀n,m:nat.S n - S m = n -m.
 // qed.
 
-theorem or_eq_eq_S: \forall n.\exists m. 
-n = (S(S O))*m \lor n = S ((S(S O))*m).
-#n (elim n)
-  ##[@ /2/
-  ##|#a #H nelim H #b#ornelim or#aeq
-    ##[@ b @ 2 //
-    ##|@ (S b) @ 1 /2/
-    ##]
-qed.
-*)
+theorem minus_O_n: ∀n:nat.0=0-n.
+#n (cases n) // qed.
 
-(******************** ordering relations ************************)
+theorem minus_n_O: ∀n:nat.n=n-0.
+#n (cases n) // qed.
 
-inductive le (n:nat) : nat → Prop ≝
-  | le_n : le n n
-  | le_S : ∀ m:nat. le n m → le n (S m).
+theorem minus_n_n: ∀n:nat.0=n-n.
+#n (elim n) // qed.
 
-interpretation "natural 'less or equal to'" 'leq x y = (le x y).
+theorem minus_Sn_n: ∀n:nat. S 0 = (S n)-n.
+#n (elim n) normalize // qed.
 
-interpretation "natural 'neither less nor equal to'" 'nleq x y = (Not (le x y)).
+theorem eq_minus_S_pred: ∀n,m. n - (S m) = pred(n -m).
+@nat_elim2 normalize // qed.
 
-definition lt: nat → nat → Prop ≝ λn,m. S n ≤ m.
+lemma plus_plus_comm_23: ∀x,y,z. x + y + z = x + z + y.
+// qed.
 
-interpretation "natural 'less than'" 'lt x y = (lt x y).
-interpretation "natural 'not less than'" 'nless x y = (Not (lt x y)).
+lemma discr_plus_xy_minus_xz: ∀x,z,y. x + y = x - z → y = 0.
+#x elim x -x // #x #IHx * normalize
+[ #y #H @(IHx 0) <minus_n_O /2 width=1/
+| #z #y >plus_n_Sm #H lapply (IHx … H) -x -z #H destruct
+]
+qed-.
 
-(* lemma eq_lt: ∀n,m. (n < m) = (S n ≤ m).
-// qed. *)
+(* Negated equalities *******************************************************)
 
-definition ge: nat → nat → Prop ≝ λn,m.m ≤ n.
+theorem not_eq_S: ∀n,m:nat. n ≠ m → S n ≠ S m.
+/2/ qed.
 
-interpretation "natural 'greater or equal to'" 'geq x y = (ge x y).
+theorem not_eq_O_S : ∀n:nat. 0 ≠ S n.
+#n @nmk #eqOS (change with (not_zero O)) >eqOS // qed.
 
-definition gt: nat → nat → Prop ≝ λn,m.m<n.
+theorem not_eq_n_Sn: ∀n:nat. n ≠ S n.
+#n (elim n) /2/ qed.
 
-interpretation "natural 'greater than'" 'gt x y = (gt x y).
-interpretation "natural 'not greater than'" 'ngtr x y = (Not (gt x y)).
+(* Atomic conclusion *******************************************************)
 
-theorem transitive_le : transitive nat le.
-#a #b #c #leab #lebc (elim lebc) /2/
-qed.
+(* not_zero *)
 
-(*
-theorem trans_le: \forall n,m,p:nat. n \leq m \to m \leq p \to n \leq p
-\def transitive_le. *)
-
-theorem transitive_lt: transitive nat lt.
-#a #b #c #ltab #ltbc (elim ltbc) /2/qed.
+theorem lt_to_not_zero : ∀n,m:nat. n < m → not_zero m.
+#n #m #Hlt (elim Hlt) // qed.
 
-(*
-theorem trans_lt: \forall n,m,p:nat. lt n m \to lt m p \to lt n p
-\def transitive_lt. *)
+(* le *)
 
 theorem le_S_S: ∀n,m:nat. n ≤ m → S n ≤ S m.
 #n #m #lenm (elim lenm) /2/ qed.
 
-theorem le_O_n : ∀n:nat. O ≤ n.
+theorem le_O_n : ∀n:nat. 0 ≤ n.
 #n (elim n) /2/ qed.
 
 theorem le_n_Sn : ∀n:nat. n ≤ S n.
 /2/ qed.
 
+theorem transitive_le : transitive nat le.
+#a #b #c #leab #lebc (elim lebc) /2/
+qed.
+
 theorem le_pred_n : ∀n:nat. pred n ≤ n.
 #n (elim n) // qed.
 
@@ -241,283 +237,19 @@ theorem le_S_S_to_le: ∀n,m:nat. S n ≤ S m → n ≤ m.
 (* demo *)
 /2/ qed-.
 
-(* this are instances of the le versions 
-theorem lt_S_S_to_lt: ∀n,m. S n < S m → n < m.
-/2/ qed. 
-
-theorem lt_to_lt_S_S: ∀n,m. n < m → S n < S m.
-/2/ qed. *)
-
-theorem lt_to_not_zero : ∀n,m:nat. n < m → not_zero m.
-#n #m #Hlt (elim Hlt) // qed.
-
-(* lt vs. le *)
-theorem not_le_Sn_O: ∀ n:nat. S n ≰ O.
-#n @nmk #Hlen0 @(lt_to_not_zero ?? Hlen0) qed.
-
-theorem not_le_to_not_le_S_S: ∀ n,m:nat. n ≰ m → S n ≰ S m.
-/3/ qed.
-
-theorem not_le_S_S_to_not_le: ∀ n,m:nat. S n ≰ S m → n ≰ m.
-/3/ qed.
-
-theorem decidable_le: ∀n,m. decidable (n≤m).
-@nat_elim2 #n /2/ #m * /3/ qed.
-
-theorem decidable_lt: ∀n,m. decidable (n < m).
-#n #m @decidable_le  qed.
-
-theorem not_le_Sn_n: ∀n:nat. S n ≰ n.
-#n (elim n) /2/ qed.
-
-(* this is le_S_S_to_le
-theorem lt_S_to_le: ∀n,m:nat. n < S m → n ≤ m.
-/2/ qed.
-*)
-
-lemma le_gen: ∀P:nat → Prop.∀n.(∀i. i ≤ n → P i) → P n.
-/2/ qed.
-
-theorem not_le_to_lt: ∀n,m. n ≰ m → m < n.
-@nat_elim2 #n
- [#abs @False_ind /2/
- |/2/
- |#m #Hind #HnotleSS @le_S_S /3/
- ]
-qed.
-
-theorem lt_to_not_le: ∀n,m. n < m → m ≰ n.
-#n #m #Hltnm (elim Hltnm) /3/ qed.
-
-theorem not_lt_to_le: ∀n,m:nat. n ≮ m → m ≤ n.
-/4/ qed.
-
-theorem le_to_not_lt: ∀n,m:nat. n ≤ m → m ≮ n.
-#n #m #H @lt_to_not_le /2/ (* /3/ *) qed.
-
-(* lt and le trans *)
-
-theorem lt_to_le_to_lt: ∀n,m,p:nat. n < m → m ≤ p → n < p.
-#n #m #p #H #H1 (elim H1) /2/ qed.
-
-theorem le_to_lt_to_lt: ∀n,m,p:nat. n ≤ m → m < p → n < p.
-#n #m #p #H (elim H) /3/ qed.
-
-theorem lt_S_to_lt: ∀n,m. S n < m → n < m.
-/2/ qed.
-
-theorem ltn_to_ltO: ∀n,m:nat. n < m → O < m.
-/2/ qed.
-
-(*
-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. *)
-
-theorem lt_O_n_elim: ∀n:nat. O < n → 
-  ∀P:nat → Prop.(∀m:nat.P (S m)) → P n.
-#n (elim n) // #abs @False_ind /2/
-qed.
-
-theorem S_pred: ∀n. 0 < n → S(pred n) = n.
-#n #posn (cases posn) //
-qed.
-
-(*
-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 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 *)
-theorem le_to_or_lt_eq: ∀n,m:nat. n ≤ m → n < m ∨ n = m.
-#n #m #lenm (elim lenm) /3/ qed.
-
-(* not eq *)
-theorem lt_to_not_eq : ∀n,m:nat. n < m → n ≠ m.
-#n #m #H @not_to_not /2/ qed.
-
-(*not lt 
-theorem 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. *)
-
-theorem not_eq_to_le_to_lt: ∀n,m. n≠m → n≤m → n<m.
-#n #m #Hneq #Hle cases (le_to_or_lt_eq ?? Hle) //
-#Heq /3/ qed-.
-(*
-nelim (Hneq Heq) qed. *)
-
-(* le elimination *)
-theorem le_n_O_to_eq : ∀n:nat. n ≤ O → O=n.
-#n (cases n) // #a  #abs @False_ind /2/ qed.
-
-theorem le_n_O_elim: ∀n:nat. n ≤ O → ∀P: nat →Prop. P O → P n.
-#n (cases n) // #a #abs @False_ind /2/ qed. 
-
-theorem 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 (elim Hle) /3/ qed.
-
-(* le and eq *)
-
-theorem le_to_le_to_eq: ∀n,m. n ≤ m → m ≤ n → n = m.
-@nat_elim2 /4/
-qed. 
-
-theorem lt_O_S : ∀n:nat. O < S n.
-/2/ qed.
-
-(*
-(* 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 *)
-
-theorem nat_elim1 : ∀n:nat.∀P:nat → Prop. 
-(∀m.(∀p. p < m → P p) → P m) → P n.
-#n #P #H 
-cut (∀q:nat. q ≤ n → P q) /2/
-(elim n) 
- [#q #HleO (* applica male *) 
-    @(le_n_O_elim ? HleO)
-    @H #p #ltpO @False_ind /2/ (* 3 *)
- |#p #Hind #q #HleS 
-    @H #a #lta @Hind @le_S_S_to_le /2/
- ]
-qed.
-
-(* some properties of functions *)
-
-definition increasing ≝ λf:nat → nat. ∀n:nat. f n < f (S n).
-
-theorem increasing_to_monotonic: ∀f:nat → nat.
-  increasing f → monotonic nat lt f.
-#f #incr #n #m #ltnm (elim ltnm) /2/
-qed.
-
-theorem le_n_fn: ∀f:nat → nat. 
-  increasing f → ∀n:nat. n ≤ f n.
-#f #incr #n (elim n) /2/
-qed-.
-
-theorem increasing_to_le: ∀f:nat → nat. 
-  increasing f → ∀m:nat.∃i.m ≤ f i.
-#f #incr #m (elim m) /2/#n * #a #lenfa
-@(ex_intro ?? (S a)) /2/
-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)
-  [@(ex_intro ? ? O) /2/
-  |#n #len * #a * #len #ltnr (cases(le_to_or_lt_eq … ltnr)) #H
-    [@(ex_intro ? ? a) % /2/ 
-    |@(ex_intro ? ? (S a)) % //
-    ]
-  ]
-qed.
-
-theorem increasing_to_injective: ∀f:nat → nat.
-  increasing f → injective nat nat f.
-#f #incr #n #m cases(decidable_le n m)
-  [#lenm cases(le_to_or_lt_eq … lenm) //
-   #lenm #eqf @False_ind @(absurd … eqf) @lt_to_not_eq 
-   @increasing_to_monotonic //
-  |#nlenm #eqf @False_ind @(absurd … eqf) @sym_not_eq 
-   @lt_to_not_eq @increasing_to_monotonic /2/
-  ]
-qed.
-
-(*********************** monotonicity ***************************)
 theorem monotonic_le_plus_r: 
 ∀n:nat.monotonic nat le (λm.n + m).
 #n #a #b (elim n) normalize //
 #m #H #leab @le_S_S /2/ qed.
 
-(*
-theorem le_plus_r: ∀p,n,m:nat. n ≤ m → p + n ≤ p + m
-≝ monotonic_le_plus_r. *)
-
 theorem monotonic_le_plus_l: 
 ∀m:nat.monotonic nat le (λn.n + m).
 /2/ qed.
 
-(*
-theorem le_plus_l: \forall p,n,m:nat. n \le m \to n + p \le m + p
-\def monotonic_le_plus_l. *)
-
 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. 
@@ -540,63 +272,12 @@ theorem le_plus_to_le: ∀a,n,m. a + n ≤ a + m → n ≤ m.
 theorem le_plus_to_le_r: ∀a,n,m. n + a ≤ m +a → n ≤ m.
 /2/ qed-. 
 
-(* plus & lt *)
-
-theorem monotonic_lt_plus_r: 
-∀n:nat.monotonic nat lt (λm.n+m).
-/2/ qed.
-
-(*
-variant lt_plus_r: \forall n,p,q:nat. p < q \to n + p < n + q \def
-monotonic_lt_plus_r. *)
-
-theorem monotonic_lt_plus_l: 
-∀n:nat.monotonic nat lt (λm.m+n).
-/2/ qed.
-
-(*
-variant lt_plus_l: \forall n,p,q:nat. p < q \to p + n < q + n \def
-monotonic_lt_plus_l. *)
-
-theorem lt_plus: ∀n,m,p,q:nat. n < m → p < q → n + p < m + q.
-#n #m #p #q #ltnm #ltpq
-@(transitive_lt ? (n+q))/2/ qed.
-
-theorem lt_plus_to_lt_l :∀n,p,q:nat. p+n < q+n → p<q.
-/2/ qed.
-
-theorem lt_plus_to_lt_r :∀n,p,q:nat. n+p < n+q → p<q.
-/2/ qed-.
-
-(*
-theorem le_to_lt_to_lt_plus: ∀a,b,c,d:nat.
-a ≤ c → b < d → a + b < c+d.
-(* bello /2/ un po' lento *)
-#a #b #c #d #leac #lebd 
-normalize napplyS le_plus // qed.
-*)
-
-(* times *)
 theorem monotonic_le_times_r: 
 ∀n:nat.monotonic nat le (λm. n * m).
 #n #x #y #lexy (elim n) normalize//(* lento /2/*)
 #a #lea @le_plus //
 qed.
 
-(*
-theorem le_times_r: \forall p,n,m:nat. n \le m \to p*n \le p*m
-\def monotonic_le_times_r. *)
-
-(*
-theorem monotonic_le_times_l: 
-∀m:nat.monotonic nat le (λn.n*m).
-/2/ qed.
-*)
-
-(*
-theorem le_times_l: \forall p,n,m:nat. n \le m \to n*p \le m*p
-\def monotonic_le_times_l. *)
-
 theorem le_times: ∀n1,n2,m1,m2:nat. 
 n1 ≤ n2  → m1 ≤ m2 → n1*m1 ≤ n2*m2.
 #n1 #n2 #m1 #m2 #len #lem @(transitive_le ? (n1*m2)) /2/
@@ -617,71 +298,67 @@ theorem le_times_to_le:
   ]
 qed-.
 
-(*
-theorem le_S_times_2: ∀n,m.O < m → n ≤ m → S n ≤ 2*m.
-#n #m #posm #lenm  (* interessante *)
-applyS (le_plus n m) // qed. *)
+theorem le_plus_minus_m_m: ∀n,m:nat. n ≤ (n-m)+m.
+#n (elim n) // #a #Hind #m (cases m) // normalize #n/2/  
+qed.
+
+theorem le_plus_to_minus_r: ∀a,b,c. a + b ≤ c → a ≤ c -b.
+#a #b #c #H @(le_plus_to_le_r … b) /2/
+qed.
+
+lemma lt_to_le: ∀x,y. x < y → x ≤ y.
+/2 width=2/ qed.
+
+lemma inv_eq_minus_O: ∀x,y. x - y = 0 → x ≤ y.
+// qed-.
+
+lemma le_x_times_x: ∀x. x ≤ x * x.
+#x elim x -x //
+qed.
+
+(* lt *)
+
+theorem transitive_lt: transitive nat lt.
+#a #b #c #ltab #ltbc (elim ltbc) /2/
+qed.
+
+theorem lt_to_le_to_lt: ∀n,m,p:nat. n < m → m ≤ p → n < p.
+#n #m #p #H #H1 (elim H1) /2/ qed.
+
+theorem le_to_lt_to_lt: ∀n,m,p:nat. n ≤ m → m < p → n < p.
+#n #m #p #H (elim H) /3/ qed.
+
+theorem lt_S_to_lt: ∀n,m. S n < m → n < m.
+/2/ qed.
+
+theorem ltn_to_ltO: ∀n,m:nat. n < m → 0 < m.
+/2/ qed.
+
+theorem lt_O_S : ∀n:nat. O < S n.
+/2/ qed.
+
+theorem monotonic_lt_plus_r: 
+∀n:nat.monotonic nat lt (λm.n+m).
+/2/ qed.
 
-(* times & lt *)
-(*
-theorem lt_O_times_S_S: ∀n,m:nat.O < (S n)*(S m).
-intros.simplify.unfold lt.apply le_S_S.apply le_O_n.
-qed. *)
+theorem monotonic_lt_plus_l: 
+∀n:nat.monotonic nat lt (λm.m+n).
+/2/ qed.
 
-(*
-theorem lt_times_eq_O: \forall a,b:nat.
-O < a → a * b = O → b = O.
-intros.
-apply (nat_case1 b)
-[ intros.
-  reflexivity
-| intros.
-  rewrite > H2 in H1.
-  rewrite > (S_pred a) in H1
-  [ apply False_ind.
-    apply (eq_to_not_lt O ((S (pred a))*(S m)))
-    [ apply sym_eq.
-      assumption
-    | apply lt_O_times_S_S
-    ]
-  | assumption
-  ]
-]
-qed. 
+theorem lt_plus: ∀n,m,p,q:nat. n < m → p < q → n + p < m + q.
+#n #m #p #q #ltnm #ltpq
+@(transitive_lt ? (n+q))/2/ qed.
 
-theorem O_lt_times_to_O_lt: \forall a,c:nat.
-O \lt (a * c) \to O \lt a.
-intros.
-apply (nat_case1 a)
-[ intros.
-  rewrite > H1 in H.
-  simplify in H.
-  assumption
-| intros.
-  apply lt_O_S
-]
-qed.
+theorem lt_plus_to_lt_l :∀n,p,q:nat. p+n < q+n → p<q.
+/2/ qed.
 
-lemma lt_times_to_lt_O: \forall i,n,m:nat. i < n*m \to O < m.
-intros.
-elim (le_to_or_lt_eq O ? (le_O_n m))
-  [assumption
-  |apply False_ind.
-   rewrite < H1 in H.
-   rewrite < times_n_O in H.
-   apply (not_le_Sn_O ? H)
-  ]
-qed. *)
+theorem lt_plus_to_lt_r :∀n,p,q:nat. n+p < n+q → p<q.
+/2/ qed-.
 
-(*
-theorem monotonic_lt_times_r: 
-∀n:nat.monotonic nat lt (λm.(S n)*m).
-/2/ 
-simplify.
-intros.elim n.
-simplify.rewrite < plus_n_O.rewrite < plus_n_O.assumption.
-apply lt_plus.assumption.assumption.
-qed. *)
+theorem increasing_to_monotonic: ∀f:nat → nat.
+  increasing f → monotonic nat lt f.
+#f #incr #n #m #ltnm (elim ltnm) /2/
+qed.
 
 theorem monotonic_lt_times_r: 
   ∀c:nat. O < c → monotonic nat lt (λt.(c*t)).
@@ -693,7 +370,7 @@ theorem monotonic_lt_times_r:
 qed.
 
 theorem monotonic_lt_times_l: 
-  ∀c:nat. O < c → monotonic nat lt (λt.(t*c)).
+  ∀c:nat. 0 < c → monotonic nat lt (λt.(t*c)).
 /2/
 qed.
 
@@ -710,87 +387,174 @@ theorem lt_times:∀n,m,p,q:nat. n<m → p<q → n*p < m*q.
 #n #m #p #q #ltnm #ltpq @lt_to_le_to_lt_times/2/
 qed.
 
-theorem lt_times_n_to_lt_l: 
-∀n,p,q:nat. p*n < q*n → p < q.
-#n #p #q #Hlt (elim (decidable_lt p q)) //
-#nltpq @False_ind @(absurd ? ? (lt_to_not_le ? ? Hlt))
-applyS monotonic_le_times_r /2/
+theorem lt_plus_to_minus_r: ∀a,b,c. a + b < c → a < c - b.
+#a #b #c #H @le_plus_to_minus_r //
 qed.
 
-theorem lt_times_n_to_lt_r: 
-∀n,p,q:nat. n*p < n*q → p < q.
+lemma lt_plus_Sn_r: ∀a,x,n. a < a + x + (S n).
+/2 width=1/ qed.
+
+theorem lt_S_S_to_lt: ∀n,m:nat. S n < S m → n < m.
+(* demo *)
 /2/ qed-.
 
-(*
-theorem nat_compare_times_l : \forall n,p,q:nat. 
-nat_compare p q = nat_compare ((S n) * p) ((S n) * q).
-intros.apply nat_compare_elim.intro.
-apply nat_compare_elim.
-intro.reflexivity.
-intro.absurd (p=q).
-apply (inj_times_r n).assumption.
-apply lt_to_not_eq. assumption.
-intro.absurd (q<p).
-apply (lt_times_to_lt_r n).assumption.
-apply le_to_not_lt.apply lt_to_le.assumption.
-intro.rewrite < H.rewrite > nat_compare_n_n.reflexivity.
-intro.apply nat_compare_elim.intro.
-absurd (p<q).
-apply (lt_times_to_lt_r n).assumption.
-apply le_to_not_lt.apply lt_to_le.assumption.
-intro.absurd (q=p).
-symmetry.
-apply (inj_times_r n).assumption.
-apply lt_to_not_eq.assumption.
-intro.reflexivity.
-qed. *)
-
-(* times and plus 
-theorem lt_times_plus_times: \forall a,b,n,m:nat. 
-a < n \to b < m \to a*m + b < n*m.
-intros 3.
-apply (nat_case n)
-  [intros.apply False_ind.apply (not_le_Sn_O ? H)
-  |intros.simplify.
-   rewrite < sym_plus.
-   unfold.
-   change with (S b+a*m1 \leq m1+m*m1).
-   apply le_plus
-    [assumption
-    |apply le_times
-      [apply le_S_S_to_le.assumption
-      |apply le_n
-      ]
+(* not le, lt *)
+
+theorem not_le_Sn_O: ∀ n:nat. S n ≰ 0.
+#n @nmk #Hlen0 @(lt_to_not_zero ?? Hlen0) qed.
+
+theorem not_le_to_not_le_S_S: ∀ n,m:nat. n ≰ m → S n ≰ S m.
+/3/ qed.
+
+theorem not_le_S_S_to_not_le: ∀ n,m:nat. S n ≰ S m → n ≰ m.
+/3/ qed.
+
+theorem not_le_Sn_n: ∀n:nat. S n ≰ n.
+#n (elim n) /2/ qed.
+
+theorem lt_to_not_le: ∀n,m. n < m → m ≰ n.
+#n #m #Hltnm (elim Hltnm) /3/ qed.
+
+theorem not_le_to_lt: ∀n,m. n ≰ m → m < n.
+@nat_elim2 #n
+ [#abs @False_ind /2/
+ |/2/
+ |#m #Hind #HnotleSS @le_S_S @Hind /2/ 
+ ]
+qed.
+
+(* not lt, le *)
+
+theorem not_lt_to_le: ∀n,m:nat. n ≮ m → m ≤ n.
+#n #m #H @le_S_S_to_le @not_le_to_lt /2/ qed.
+
+theorem le_to_not_lt: ∀n,m:nat. n ≤ m → m ≮ n.
+#n #m #H @lt_to_not_le /2/ (* /3/ *) qed.
+
+(* Compound conclusion ******************************************************)
+
+theorem decidable_eq_nat : ∀n,m:nat.decidable (n=m).
+@nat_elim2 #n [ (cases n) /2/ | /3/ | #m #Hind (cases Hind) /3/]
+qed. 
+
+theorem decidable_le: ∀n,m. decidable (n≤m).
+@nat_elim2 #n /2/ #m * /3/ qed.
+
+theorem decidable_lt: ∀n,m. decidable (n < m).
+#n #m @decidable_le  qed.
+
+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)
+  [@(ex_intro ? ? O) /2/
+  |#n #len * #a * #len #ltnr (cases(le_to_or_lt_eq … ltnr)) #H
+    [@(ex_intro ? ? a) % /2/ 
+    |@(ex_intro ? ? (S a)) % //
     ]
   ]
-qed. *)
+qed.
 
-(************************** minus ******************************)
+lemma le_inv_plus_l: ∀x,y,z. x + y ≤ z → x ≤ z - y ∧ y ≤ z.
+/3/ qed-.
 
-let 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).
+lemma lt_inv_plus_l: ∀x,y,z. x + y < z → x < z ∧ y < z - x.
+/3/ qed-.
 
-theorem minus_S_S: ∀n,m:nat.S n - S m = n -m.
-// qed.
+lemma lt_or_ge: ∀m,n. m < n ∨ n ≤ m.
+#m #n elim (decidable_lt m n) /2/ /3/
+qed-.
 
-theorem minus_O_n: ∀n:nat.O=O-n.
-#n (cases n) // qed.
+lemma le_or_ge: ∀m,n. m ≤ n ∨ n ≤ m.
+#m #n elim (decidable_le m n) /2/ /4/
+qed-.
 
-theorem minus_n_O: ∀n:nat.n=n-O.
-#n (cases n) // qed.
+(* More general conclusion **************************************************)
 
-theorem minus_n_n: ∀n:nat.O=n-n.
-#n (elim n) // qed.
+theorem nat_ind_plus: ∀R:predicate nat.
+                      R 0 → (∀n. R n → R (n + 1)) → ∀n. R n.
+/3 by nat_ind/ qed-.
 
-theorem minus_Sn_n: ∀n:nat. S O = (S n)-n.
-#n (elim n) normalize // qed.
+theorem lt_O_n_elim: ∀n:nat. 0 < n → 
+  ∀P:nat → Prop.(∀m:nat.P (S m)) → P n.
+#n (elim n) // #abs @False_ind /2/ @absurd
+qed.
+
+theorem le_n_O_elim: ∀n:nat. n ≤ O → ∀P: nat →Prop. P O → P n.
+#n (cases n) // #a #abs @False_ind /2/ qed. 
+
+theorem 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 (elim Hle) /3/ qed.
+
+theorem nat_elim1 : ∀n:nat.∀P:nat → Prop. 
+(∀m.(∀p. p < m → P p) → P m) → P n.
+#n #P #H 
+cut (∀q:nat. q ≤ n → P q) /2/
+(elim n) 
+ [#q #HleO (* applica male *) 
+    @(le_n_O_elim ? HleO)
+    @H #p #ltpO @False_ind /2/ (* 3 *)
+ |#p #Hind #q #HleS 
+    @H #a #lta @Hind @le_S_S_to_le /2/
+ ]
+qed.
+
+fact f_ind_aux: ∀A. ∀f:A→ℕ. ∀P:predicate A.
+                (∀n. (∀a. f a < n → P a) → ∀a. f a = n → P a) →
+                ∀n,a. f a = n → P a.
+#A #f #P #H #n @(nat_elim1 … n) -n #n /3 width=3/ (**) (* auto slow (34s) without #n *)
+qed-.
+
+lemma f_ind: ∀A. ∀f:A→ℕ. ∀P:predicate A.
+             (∀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 **************************************************)
+
+theorem lt_to_not_eq : ∀n,m:nat. n < m → n ≠ m.
+#n #m #H @not_to_not /2/ qed.
+
+(* More equalities **********************************************************)
+
+theorem le_n_O_to_eq : ∀n:nat. n ≤ 0 → 0=n.
+#n (cases n) // #a  #abs @False_ind /2/ qed.
+
+theorem le_to_le_to_eq: ∀n,m. n ≤ m → m ≤ n → n = m.
+@nat_elim2 /4 by le_n_O_to_eq, monotonic_pred, eq_f, sym_eq/
+qed. 
+
+theorem increasing_to_injective: ∀f:nat → nat.
+  increasing f → injective nat nat f.
+#f #incr #n #m cases(decidable_le n m)
+  [#lenm cases(le_to_or_lt_eq … lenm) //
+   #lenm #eqf @False_ind @(absurd … eqf) @lt_to_not_eq 
+   @increasing_to_monotonic //
+  |#nlenm #eqf @False_ind @(absurd … eqf) @sym_not_eq 
+   @lt_to_not_eq @increasing_to_monotonic /2/
+  ]
+qed.
 
 theorem minus_Sn_m: ∀m,n:nat. m ≤ n → S n -m = S (n-m).
 (* qualcosa da capire qui 
@@ -802,17 +566,6 @@ theorem minus_Sn_m: ∀m,n:nat. m ≤ n → S n -m = S (n-m).
   ]
 qed.
 
-(*
-theorem not_eq_to_le_to_le_minus: 
-  ∀n,m.n ≠ m → n ≤ m → n ≤ m - 1.
-#n * #m (cases m// #m normalize
-#H #H1 napply le_S_S_to_le
-napplyS (not_eq_to_le_to_lt n (S m) H H1)
-qed. *)
-
-theorem eq_minus_S_pred: ∀n,m. n - (S m) = pred(n -m).
-@nat_elim2 normalize // qed.
-
 theorem plus_minus:
 ∀m,n,p:nat. m ≤ n → (n-m)+p = (n+p)-m.
 @nat_elim2 
@@ -829,10 +582,6 @@ theorem plus_minus_m_m: ∀n,m:nat.
   m ≤ n → n = (n-m)+m.
 #n #m #lemn @sym_eq /2/ qed.
 
-theorem le_plus_minus_m_m: ∀n,m:nat. n ≤ (n-m)+m.
-#n (elim n) // #a #Hind #m (cases m) // normalize #n/2/  
-qed.
-
 theorem minus_to_plus :∀n,m,p:nat.
   m ≤ n → n-m = p → n = m+p.
 #n #m #p #lemn #eqp (applyS plus_minus_m_m) //
@@ -847,57 +596,17 @@ pred n - pred m = n - m.
 #n #m #posn #posm @(lt_O_n_elim n posn) @(lt_O_n_elim m posm) //.
 qed.
 
+theorem plus_minus_commutative: ∀x,y,z. z ≤ y → x + (y - z) = x + y - z.
+/2 by plus_minus/ 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.
+(* More atomic conclusion ***************************************************)
 
-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.
-*)
+(* le *)
 
-(* monotonicity and galois *)
+theorem le_n_fn: ∀f:nat → nat. 
+  increasing f → ∀n:nat. n ≤ f n.
+#f #incr #n (elim n) /2/
+qed-.
 
 theorem monotonic_le_minus_l: 
 ∀p,q,n:nat. q ≤ p → q-n ≤ p-n.
@@ -920,10 +629,43 @@ qed.
 theorem le_plus_to_minus: ∀n,m,p. n ≤ p+m → n-m ≤ p.
 #n #m #p #lep /2/ qed.
 
-theorem le_plus_to_minus_r: ∀a,b,c. a + b ≤ c → a ≤ c -b.
-#a #b #c #H @(le_plus_to_le_r … b) /2/
+theorem monotonic_le_minus_r: 
+∀p,q,n:nat. q ≤ p → n-p ≤ n-q.
+#p #q #n #lepq @le_plus_to_minus
+@(transitive_le … (le_plus_minus_m_m ? q)) /2/
+qed.
+
+theorem increasing_to_le: ∀f:nat → nat. 
+  increasing f → ∀m:nat.∃i.m ≤ f i.
+#f #incr #m (elim m) /2/#n * #a #lenfa
+@(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.
+#x1 #y1 #x2 #y2 #H1 #H2 /2/ @le_plus // /2/ /3 by le_minus_to_plus, monotonic_le_plus_r, transitive_le/ qed.
+*)
+
+lemma minus_le: ∀x,y. x - y ≤ x.
+/2 width=1/ qed.
+
+(* lt *)
+
+theorem not_eq_to_le_to_lt: ∀n,m. n≠m → n≤m → n<m.
+#n #m #Hneq #Hle cases (le_to_or_lt_eq ?? Hle) //
+#Heq @not_le_to_lt /2/ qed-.
+
+theorem lt_times_n_to_lt_l: 
+∀n,p,q:nat. p*n < q*n → p < q.
+#n #p #q #Hlt (elim (decidable_lt p q)) //
+#nltpq @False_ind @(absurd ? ? (lt_to_not_le ? ? Hlt))
+applyS monotonic_le_times_r /2/
 qed.
 
+theorem lt_times_n_to_lt_r: 
+∀n,p,q:nat. n*p < n*q → p < q.
+/2/ qed-.
+
 theorem lt_minus_to_plus: ∀a,b,c. a - b < c → a < c + b.
 #a #b #c #H @not_le_to_lt 
 @(not_to_not … (lt_to_not_le …H)) /2/
@@ -938,21 +680,25 @@ theorem lt_plus_to_minus: ∀n,m,p. m ≤ n → n < p+m → n-m < p.
 #n #m #p #lenm #H normalize <minus_Sn_m // @le_plus_to_minus //
 qed.
 
-theorem lt_plus_to_minus_r: ∀a,b,c. a + b < c → a < c - b.
-#a #b #c #H @le_plus_to_minus_r //
-qed. 
-
-theorem monotonic_le_minus_r: 
-∀p,q,n:nat. q ≤ p → n-p ≤ n-q.
-#p #q #n #lepq @le_plus_to_minus
-@(transitive_le … (le_plus_minus_m_m ? q)) /2/
-qed.
-
 theorem monotonic_lt_minus_l: ∀p,q,n. n ≤ q → q < p → q - n < p - n.
 #p #q #n #H1 #H2
 @lt_plus_to_minus_r <plus_minus_m_m //
 qed.
 
+(* More compound conclusion *************************************************)
+
+lemma discr_minus_x_xy: ∀x,y. x = x - y → x = 0 ∨ y = 0.
+* /2 width=1/ #x * /2 width=1/ #y normalize #H 
+lapply (minus_le x y) <H -H #H
+elim (not_le_Sn_n x) #H0 elim (H0 ?) //
+qed-.
+
+lemma plus_le_0: ∀x,y. x + y ≤ 0 → x = 0 ∧ y = 0.
+#x #y #H elim (le_inv_plus_l … H) -H #H1 #H2 /3 width=1/
+qed-.
+
+(* Still more equalities ****************************************************)
+
 theorem eq_minus_O: ∀n,m:nat.
   n ≤ m → n-m = O.
 #n #m #lenm @(le_n_O_elim (n-m)) /2/
@@ -961,7 +707,7 @@ qed.
 theorem distributive_times_minus: distributive ? times minus.
 #a #b #c
 (cases (decidable_lt b c)) #Hbc
- [> eq_minus_O /2/ >eq_minus_O // 
+ [> eq_minus_O [2:/2/] >eq_minus_O // 
   @monotonic_le_times_r /2/
  |@sym_eq (applyS plus_to_minus) <distributive_times_plus 
   @eq_f (applyS plus_minus_m_m) /2/
@@ -977,12 +723,6 @@ cases (decidable_le (m+p) n) #Hlt
   ]
 qed.
 
-(*
-theorem plus_minus: ∀n,m,p. p ≤ m → (n+m)-p = n +(m-p).
-#n #m #p #lepm @plus_to_minus >(commutative_plus p)
->associative_plus <plus_minus_m_m //
-qed.  *)
-
 theorem minus_minus: ∀n,m,p:nat. p ≤ m → m ≤ n →
   p+(n-m) = n-(m-p).
 #n #m #p #lepm #lemn
@@ -990,7 +730,31 @@ theorem minus_minus: ∀n,m,p:nat. p ≤ m → m ≤ n →
 <commutative_plus <plus_minus_m_m //
 qed.
 
+lemma minus_minus_comm: ∀a,b,c. a - b - c = a - c - b.
+/3 by monotonic_le_minus_l, le_to_le_to_eq/ qed.
+
+lemma minus_le_minus_minus_comm: ∀b,c,a. c ≤ b → a - (b - c) = a + c - b.
+#b #c #a #H >(plus_minus_m_m b c) in ⊢ (? ? ?%); //
+qed.
+
+lemma minus_minus_m_m: ∀m,n. n ≤ m → m - (m - n) = n.
+/2 width=1/ qed.
+
+lemma minus_plus_plus_l: ∀x,y,h. (x + h) - (y + h) = x - y.
+// qed.
+
+(* Stilll more atomic conclusion ********************************************)
+
+(* le *)
+
+lemma le_fwd_plus_plus_ge: ∀m1,m2. m2 ≤ m1 → ∀n1,n2. m1 + n1 ≤ m2 + n2 → n1 ≤ n2.
+#m1 #m2 #H #n1 #n2 >commutative_plus
+#H elim (le_inv_plus_l … H) -H >commutative_plus <minus_le_minus_minus_comm //
+#H #_ @(transitive_le … H) /2 width=1/
+qed-. 
+
 (*********************** boolean arithmetics ********************) 
+
 include "basics/bool.ma".
 
 let rec eqb n m ≝ 
@@ -1058,37 +822,12 @@ theorem not_le_to_leb_false: ∀n,m. n ≰ m → leb n m = false.
 theorem lt_to_leb_false: ∀n,m. m < n → leb n m = false.
 /3/ qed.
 
-(* serve anche ltb? 
-ndefinition ltb ≝λn,m. leb (S n) m.
-
-theorem ltb_elim: ∀n,m:nat. ∀P:bool → Prop. 
-(n < m → P true) → (n ≮ m → P false) → P (ltb n m).
-#n #m #P #Hlt #Hnlt
-napply leb_elim /3/ qed.
-
-theorem ltb_true_to_lt:∀n,m.ltb n m = true → n < m.
-#n #m #Hltb napply leb_true_to_le nassumption
-qed.
-
-theorem ltb_false_to_not_lt:∀n,m.
-  ltb n m = false → n ≮ m.
-#n #m #Hltb napply leb_false_to_not_le nassumption
-qed.
-
-theorem lt_to_ltb_true: ∀n,m. n < m → ltb n m = true.
-#n #m #Hltb napply le_to_leb_true nassumption
-qed.
-
-theorem le_to_ltb_false: ∀n,m. m \le n → ltb n m = false.
-#n #m #Hltb napply lt_to_leb_false /2/
-qed. *)
-
 (* min e max *)
 definition min: nat →nat →nat ≝
-λn.λm. if_then_else ? (leb n m) n m.
+λn.λm. if leb n m then n else m.
 
 definition max: nat →nat →nat ≝
-λn.λm. if_then_else ? (leb n m) m n.
+λn.λm. if leb n m then m else n.
 
 lemma commutative_min: commutative ? min.
 #n #m normalize @leb_elim 
@@ -1121,4 +860,3 @@ lemma le_maxr: ∀i,n,m. max n m ≤ i → m ≤ i.
 lemma to_max: ∀i,n,m. n ≤ i → m ≤ i → max n m ≤ i.
 #i #n #m #leni #lemi normalize (cases (leb n m)) 
 normalize // qed.
-