(* *)
(**************************************************************************)
-include "arithmetics/nat.ma".
+include "basics/star.ma".
+include "basics/lists/list.ma".
+include "arithmetics/exp.ma".
include "xoa_notation.ma".
include "xoa.ma".
+(* logic *)
+
(* Note: For some reason this cannot be in the standard library *)
interpretation "logical false" 'false = False.
non associative with precedence 90
for @{'false}.
+(* relations *)
+
+definition confluent1: ∀A. relation A → predicate A ≝ λA,R,a0.
+ ∀a1. R a0 a1 → ∀a2. R a0 a2 →
+ ∃∃a. R a1 a & R a2 a.
+
+(* Note: confluent1 would be redundant if \Pi-reduction where enabled *)
+definition confluent: ∀A. predicate (relation A) ≝ λA,R.
+ ∀a0. confluent1 … R a0.
+
+(* booleans *)
+
+definition is_false: predicate bool ≝ λb.
+ false = b.
+
+(* arithmetics *)
+
lemma lt_refl_false: ∀n. n < n → ⊥.
#n #H elim (lt_to_not_eq … H) -H /2 width=1/
qed-.
lemma lt_zero_false: ∀n. n < 0 → ⊥.
#n #H elim (lt_to_not_le … H) -H /2 width=1/
qed-.
+
+lemma plus_lt_false: ∀m,n. m + n < m → ⊥.
+#m #n #H elim (lt_to_not_le … H) -H /2 width=1/
+qed-.
+
+lemma lt_or_eq_or_gt: ∀m,n. ∨∨ m < n | n = m | n < m.
+#m #n elim (lt_or_ge m n) /2 width=1/
+#H elim H -m /2 width=1/
+#m #Hm * #H /2 width=1/ /3 width=1/
+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.
+
+(* lists *)
+
+(* Note: notation for nil not involving brackets *)
+notation > "◊"
+ non associative with precedence 90
+ for @{'nil}.