mk: sortedo — list is non-decreasing (intarith)

Three conde clauses: empty list / singleton list / two-or-more (where
the first two satisfy lteo-i and the rest is recursively sorted). Uses
ground-only integer comparison (intarith), so the input list must
walk to ground integers.

7 new tests, 463/463 cumulative.

lib/minikanren/intarith.sx

@@ -68,3 +68,12 @@
 (define
   odd-i
   (fn (n) (project (n) (if (and (number? n) (odd? n)) succeed fail))))
+
+(define
+  sortedo
+  (fn
+    (l)
+    (conde
+      ((nullo l))
+      ((fresh (a) (== l (list a))))
+      ((fresh (a b rest mid) (conso a mid l) (conso b rest mid) (lteo-i a b) (sortedo mid))))))

lib/minikanren/tests/sortedo.sx

@@ -0,0 +1,40 @@
+;; lib/minikanren/tests/sortedo.sx — checks list is non-decreasing.
+
+(mk-test
+  "sortedo-empty"
+  (run* q (sortedo (list)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "sortedo-singleton"
+  (run* q (sortedo (list 42)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "sortedo-ascending"
+  (run* q (sortedo (list 1 2 3 4)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "sortedo-with-equal-adjacent"
+  (run*
+    q
+    (sortedo (list 1 1 2 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "sortedo-out-of-order-fails"
+  (run* q (sortedo (list 1 3 2)))
+  (list))
+
+(mk-test
+  "sortedo-descending-fails"
+  (run* q (sortedo (list 3 2 1)))
+  (list))
+
+(mk-test
+  "sortedo-pair-equal"
+  (run* q (sortedo (list 5 5)))
+  (list (make-symbol "_.0")))
+
+(mk-tests-run!)