- natural numbers with infinity for lambdadelta

[helm.git] / matita / matita / contribs / lambdadelta / ground_2 / ynat / ynat_succ.ma

diff --git a/matita/matita/contribs/lambdadelta/ground_2/ynat/ynat_succ.ma b/matita/matita/contribs/lambdadelta/ground_2/ynat/ynat_succ.ma
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+(**************************************************************************)
+(*       ___                                                              *)
+(*      ||M||                                                             *)
+(*      ||A||       A project by Andrea Asperti                           *)
+(*      ||T||                                                             *)
+(*      ||I||       Developers:                                           *)
+(*      ||T||         The HELM team.                                      *)
+(*      ||A||         http://helm.cs.unibo.it                             *)
+(*      \   /                                                             *)
+(*       \ /        This file is distributed under the terms of the       *)
+(*        v         GNU General Public License Version 2                  *)
+(*                                                                        *)
+(**************************************************************************)
+
+include "ground_2/notation/functions/successor_1.ma".
+include "ground_2/ynat/ynat_pred.ma".
+
+(* NATURAL NUMBERS WITH INFINITY ********************************************)
+
+(* the successor function *)
+definition ysucc: ynat → ynat ≝ λm. match m with
+[ yinj m ⇒ S m
+| Y      ⇒ Y
+].
+
+interpretation "ynat successor" 'Successor m = (ysucc m).
+
+(* Properties ***************************************************************)
+
+lemma ypred_succ: ∀m. ⫰⫯m = m.
+* // qed.
+
+(* Inversion lemmas *********************************************************)
+
+lemma ysucc_inj: ∀m,n. ⫯m = ⫯n → m = n.
+#m #n #H <(ypred_succ m) <(ypred_succ n) //
+qed-.
+
+lemma ysucc_inv_refl: ∀m. ⫯m = m → m = ∞.
+* // normalize
+#m #H lapply (yinj_inj … H) -H (**) (* destruct lemma needed *)
+#H elim (lt_refl_false m) //
+qed-.
+
+lemma ysucc_inv_inj_sn: ∀m2,n1. yinj m2 = ⫯n1 →
+                        ∃∃m1. n1 = yinj m1 & m2 = S m1.
+#m2 * normalize
+[ #n1 #H destruct /2 width=3 by ex2_intro/
+| #H destruct
+]
+qed-.
+
+lemma ysucc_inv_inj_dx: ∀m2,n1. ⫯n1 = yinj m2  →
+                        ∃∃m1. n1 = yinj m1 & m2 = S m1.
+/2 width=1 by ysucc_inv_inj_sn/ qed-.