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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 "basics/relations.ma".
+include "ground_2/xoa/xoa_props.ma".
+
+(* GENERIC RELATIONS ********************************************************)
+
+(* PROPERTIES OF RELATIONS **************************************************)
+
+definition relation5 : Type[0] → Type[0] → Type[0] → Type[0] → Type[0] → Type[0]
+≝ λA,B,C,D,E.A→B→C→D→E→Prop.
+
+definition relation6 : Type[0] → Type[0] → Type[0] → Type[0] → Type[0] → Type[0] → Type[0]
+≝ λA,B,C,D,E,F.A→B→C→D→E→F→Prop.
+
+definition Decidable: Prop → Prop ≝ λR. R ∨ (R → ⊥).
+
+definition Transitive: ∀A. ∀R: relation A. Prop ≝ λA,R.
+                       ∀a1,a0. R a1 a0 → ∀a2. R a0 a2 → R a1 a2.
+
+definition left_cancellable: ∀A. ∀R: relation A. Prop ≝ λA,R.
+                             ∀a0,a1. R a0 a1 → ∀a2. R a0 a2 → R a1 a2.
+
+definition right_cancellable: ∀A. ∀R: relation A. Prop ≝ λA,R.
+                              ∀a1,a0. R a1 a0 → ∀a2. R a2 a0 → R a1 a2.
+
+definition pw_confluent2: ∀A. relation A → relation A → predicate A ≝ λA,R1,R2,a0.
+                          ∀a1. R1 a0 a1 → ∀a2. R2 a0 a2 →
+                          ∃∃a. R2 a1 a & R1 a2 a.
+
+definition confluent2: ∀A. relation (relation A) ≝ λA,R1,R2.
+                       ∀a0. pw_confluent2 A R1 R2 a0.
+
+definition transitive2: ∀A. ∀R1,R2: relation A. Prop ≝ λA,R1,R2.
+                        ∀a1,a0. R1 a1 a0 → ∀a2. R2 a0 a2 →
+                        ∃∃a. R2 a1 a & R1 a a2.
+
+definition bi_confluent:  ∀A,B. ∀R: bi_relation A B. Prop ≝ λA,B,R.
+                          ∀a0,a1,b0,b1. R a0 b0 a1 b1 → ∀a2,b2. R a0 b0 a2 b2 →
+                          ∃∃a,b. R a1 b1 a b & R a2 b2 a b.
+
+definition lsub_trans: ∀A,B. relation2 (A→relation B) (relation A) ≝ λA,B,R1,R2.
+                       ∀L2,T1,T2. R1 L2 T1 T2 → ∀L1. R2 L1 L2 → R1 L1 T1 T2.
+
+definition s_r_confluent1: ∀A,B. relation2 (A→relation B) (B→relation A) ≝ λA,B,R1,R2.
+                           ∀L1,T1,T2. R1 L1 T1 T2 → ∀L2. R2 T1 L1 L2 → R2 T2 L1 L2.
+
+definition is_mono: ∀B:Type[0]. predicate (predicate B) ≝ 
+                    λB,R. ∀b1. R b1 → ∀b2. R b2 → b1 = b2.
+
+definition is_inj2: ∀A,B:Type[0]. predicate (relation2 A B) ≝ 
+                    λA,B,R. ∀a1,b. R a1 b → ∀a2. R a2 b → a1 = a2.
+
+(* Normal form and strong normalization *************************************)
+
+definition NF: ∀A. relation A → relation A → predicate A ≝
+   λA,R,S,a1. ∀a2. R a1 a2 → S a1 a2.
+
+definition NF_dec: ∀A. relation A → relation A → Prop ≝
+                   λA,R,S. ∀a1. NF A R S a1 ∨
+                   ∃∃a2. R … a1 a2 & (S a1 a2 → ⊥).
+
+inductive SN (A) (R,S:relation A): predicate A ≝
+| SN_intro: ∀a1. (∀a2. R a1 a2 → (S a1 a2 → ⊥) → SN A R S a2) → SN A R S a1
+.
+
+lemma NF_to_SN: ∀A,R,S,a. NF A R S a → SN A R S a.
+#A #R #S #a1 #Ha1
+@SN_intro #a2 #HRa12 #HSa12
+elim HSa12 -HSa12 /2 width=1 by/
+qed.
+
+definition NF_sn: ∀A. relation A → relation A → predicate A ≝
+   λA,R,S,a2. ∀a1. R a1 a2 → S a1 a2.
+
+inductive SN_sn (A) (R,S:relation A): predicate A ≝
+| SN_sn_intro: ∀a2. (∀a1. R a1 a2 → (S a1 a2 → ⊥) → SN_sn A R S a1) → SN_sn A R S a2
+.
+
+lemma NF_to_SN_sn: ∀A,R,S,a. NF_sn A R S a → SN_sn A R S a.
+#A #R #S #a2 #Ha2
+@SN_sn_intro #a1 #HRa12 #HSa12
+elim HSa12 -HSa12 /2 width=1 by/
+qed.