definition Decidable: Prop → Prop ≝ λR. R ∨ (R → ⊥).
+definition Confluent: ∀A. ∀R: relation A. Prop ≝ λA,R.
+ ∀a0,a1. R a0 a1 → ∀a2. R a0 a2 →
+ ∃∃a. R a1 a & R a2 a.
+
+definition Transitive: ∀A. ∀R: relation A. Prop ≝ λA,R.
+ ∀a1,a0. R a1 a0 → ∀a2. R a0 a2 → R a1 a2.
+
definition confluent2: ∀A. ∀R1,R2: relation A. Prop ≝ λA,R1,R2.
∀a0,a1. R1 a0 a1 → ∀a2. R2 a0 a2 →
∃∃a. R2 a1 a & R1 a2 a.
@SN_intro #a2 #HRa12 #HSa12
elim (HSa12 ?) -HSa12 /2 width=1/
qed.
+
+definition NF_sn: ∀A. relation A → relation A → predicate A ≝
+ λA,R,S,a2. ∀a1. R a1 a2 → S a2 a1.
+
+inductive SN_sn (A) (R,S:relation A): predicate A ≝
+| SN_sn_intro: ∀a2. (∀a1. R a1 a2 → (S a2 a1 → ⊥) → 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/
+qed.