update in lambdadelta

[helm.git] / matita / matita / contribs / lambdadelta / ground_2A / ynat / ynat_min.ma

diff --git a/matita/matita/contribs/lambdadelta/ground_2A/ynat/ynat_min.ma b/matita/matita/contribs/lambdadelta/ground_2A/ynat/ynat_min.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_2A/ynat/ynat_plus.ma".
+
+(* NATURAL NUMBERS WITH INFINITY ********************************************)
+
+fact ymin_pre_dx_aux: ∀x,y. y ≤ x → x - (x - y) ≤ y.
+#x #y * -x -y
+[ #x #y #Hxy >yminus_inj
+ /3 width=4 by yle_inj, monotonic_le_minus_l/
+| * //
+]
+qed-.
+
+lemma ymin_pre_sn: ∀x,y. x ≤ y → x - (x - y) = x.
+#x #y * -x -y //
+#x #y #Hxy >yminus_inj >(eq_minus_O … Hxy) -Hxy //
+qed-.
+
+lemma ymin_pre_i_dx: ∀x,y. x - (x - y) ≤ y.
+#x #y elim (yle_split x y) /2 width=1 by ymin_pre_dx_aux/
+#Hxy >(ymin_pre_sn … Hxy) //
+qed.
+
+lemma ymin_pre_i_sn: ∀x,y. x - (x - y) ≤ x.
+// qed.
+
+lemma ymin_pre_dx: ∀x,y. y ≤ yinj x → yinj x - (yinj x - y) = y.
+#x #y #H elim (yle_inv_inj2 … H) -H
+#z #Hzx #H destruct >yminus_inj
+/3 width=4 by minus_le_minus_minus_comm, eq_f/
+qed-.
+
+lemma ymin_pre_e: ∀z,x. z ≤ yinj x → ∀y. z ≤ y →
+                  z ≤ yinj x - (yinj x - y).
+#z #x #Hzx #y #Hzy elim (yle_split x y)
+[ #H >(ymin_pre_sn … H) -y //
+| #H >(ymin_pre_dx … H) -x //
+]
+qed.