--- /dev/null
+(**************************************************************************)
+(* ___ *)
+(* ||M|| *)
+(* ||A|| A project by Andrea Asperti *)
+(* ||T|| *)
+(* ||I|| Developers: *)
+(* ||T|| A.Asperti, C.Sacerdoti Coen, *)
+(* ||A|| E.Tassi, S.Zacchiroli *)
+(* \ / *)
+(* \ / This file is distributed under the terms of the *)
+(* v GNU Lesser General Public License Version 2.1 *)
+(* *)
+(**************************************************************************)
+
+set "baseuri" "cic:/matita/nat/nat".
+
+include "higher_order_defs/functions.ma".
+
+inductive nat : Set \def
+ | O : nat
+ | S : nat \to nat.
+
+definition pred: nat \to nat \def
+\lambda n:nat. match n with
+[ O \Rightarrow O
+| (S p) \Rightarrow p ].
+
+theorem pred_Sn : \forall n:nat.n=(pred (S n)).
+intros. reflexivity.
+qed.
+
+theorem injective_S : injective nat nat S.
+unfold injective.
+intros.
+rewrite > pred_Sn.
+rewrite > (pred_Sn y).
+apply eq_f. assumption.
+qed.
+
+theorem inj_S : \forall n,m:nat.(S n)=(S m) \to n=m
+\def injective_S.
+
+theorem not_eq_S : \forall n,m:nat.
+\lnot n=m \to S n \neq S m.
+intros. unfold Not. intros.
+apply H. apply injective_S. assumption.
+qed.
+
+definition not_zero : nat \to Prop \def
+\lambda n: nat.
+ match n with
+ [ O \Rightarrow False
+ | (S p) \Rightarrow True ].
+
+theorem not_eq_O_S : \forall n:nat. O \neq S n.
+intros. unfold Not. intros.
+cut (not_zero O).
+exact Hcut.
+rewrite > H.exact I.
+qed.
+
+theorem not_eq_n_Sn : \forall n:nat. n \neq S n.
+intros.elim n.
+apply not_eq_O_S.
+apply not_eq_S.assumption.
+qed.
+
+theorem nat_case:
+\forall n:nat.\forall P:nat \to Prop.
+P O \to (\forall m:nat. P (S m)) \to P n.
+intros.elim n.assumption.apply H1.
+qed.
+
+theorem nat_case1:
+\forall n:nat.\forall P:nat \to Prop.
+(n=O \to P O) \to (\forall m:nat. (n=(S m) \to P (S m))) \to P n.
+intros 2.elim n.
+apply H.reflexivity.
+apply H2.reflexivity.
+qed.
+
+theorem nat_elim2 :
+\forall R:nat \to nat \to Prop.
+(\forall n:nat. R O n) \to
+(\forall n:nat. R (S n) O) \to
+(\forall n,m:nat. R n m \to R (S n) (S m)) \to \forall n,m:nat. R n m.
+intros 5.elim n.
+apply H.
+apply (nat_case m).apply H1.
+intros.apply H2. apply H3.
+qed.
+
+theorem decidable_eq_nat : \forall n,m:nat.decidable (n=m).
+intros.unfold decidable.
+apply (nat_elim2 (\lambda n,m.(Or (n=m) ((n=m) \to False)))).
+intro.elim n1.
+left.reflexivity.
+right.apply not_eq_O_S.
+intro.right.intro.
+apply (not_eq_O_S n1).
+apply sym_eq.assumption.
+intros.elim H.
+left.apply eq_f. assumption.
+right.intro.apply H1.apply inj_S.assumption.
+qed.
+