X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=blobdiff_plain;f=matita%2Fmatita%2Fcontribs%2Flambda_delta%2Fground_2%2Farith.ma;h=8afc445b8f4343d68494ce3e774ebc8beb196c9c;hb=bde429ac54e48de74b3d8b1df72dfcb86aa9bae5;hp=ad3ab4a3d5dab681e6971977d0d3e078251275ec;hpb=53f874fba5b9c39a788085515a4fefe5d29281da;p=helm.git

diff --git a/matita/matita/contribs/lambda_delta/ground_2/arith.ma b/matita/matita/contribs/lambda_delta/ground_2/arith.ma
index ad3ab4a3d..8afc445b8 100644
--- a/matita/matita/contribs/lambda_delta/ground_2/arith.ma
+++ b/matita/matita/contribs/lambda_delta/ground_2/arith.ma
@@ -44,7 +44,7 @@ lemma arith_h1: ∀a1,a2,b,c1. c1 ≤ a1 → c1 ≤ b →
 #a1 #a2 #b #c1 #H1 #H2 >plus_minus // /2 width=1/
 qed.
 
-(* inversion & forward lemmas ***********************************************)
+(* Inversion & forward lemmas ***********************************************)
 
 axiom eq_nat_dec: ∀n1,n2:nat. Decidable (n1 = n2).
 
@@ -80,7 +80,7 @@ qed-.
 lemma plus_xySz_x_false: ∀z,x,y. x + y + S z = x → ⊥.
 /2 width=4 by le_plus_xySz_x_false/ qed-.
 
-(* iterators ****************************************************************)
+(* Iterators ****************************************************************)
 
 (* Note: see also: lib/arithemetcs/bigops.ma *)
 let rec iter (n:nat) (B:Type[0]) (op: B → B) (nil: B) ≝
@@ -98,3 +98,30 @@ qed.
 lemma iter_n_Sm: ∀B:Type[0]. ∀f:B→B. ∀b,l. f^l (f b) = f (f^l b).
 #B #f #b #l elim l -l normalize //
 qed.
+
+(* Trichotomy operator ******************************************************)
+
+(* Note: this is "if eqb n1 n2 then a2 else if leb n1 n2 then a1 else a3" *)
+let rec tri (A:Type[0]) n1 n2 a1 a2 a3 on n1 : A ≝
+  match n1 with 
+  [ O    ⇒ match n2 with [ O ⇒ a2 | S n2 ⇒ a1 ]
+  | S n1 ⇒ match n2 with [ O ⇒ a3 | S n2 ⇒ tri A n1 n2 a1 a2 a3 ]
+  ].
+
+lemma tri_lt: ∀A,a1,a2,a3,n2,n1. n1 < n2 → tri A n1 n2 a1 a2 a3 = a1.
+#A #a1 #a2 #a3 #n2 elim n2 -n2
+[ #n1 #H elim (lt_zero_false … H)
+| #n2 #IH #n1 elim n1 -n1 // /3 width=1/
+]
+qed.
+
+lemma tri_eq: ∀A,a1,a2,a3,n. tri A n n a1 a2 a3 = a2.
+#A #a1 #a2 #a3 #n elim n -n normalize //
+qed.
+
+lemma tri_gt: ∀A,a1,a2,a3,n1,n2. n2 < n1 → tri A n1 n2 a1 a2 a3 = a3.
+#A #a1 #a2 #a3 #n1 elim n1 -n1
+[ #n2 #H elim (lt_zero_false … H)
+| #n1 #IH #n2 elim n2 -n2 // /3 width=1/
+]
+qed.