mk: selecto — choose element + rest of list

Classic miniKanren relation. (selecto x rest l) holds when l contains
x at any position with `rest` being everything else. Direct base case
(l = (x . rest)) plus the skip-head recursion that threads the head
through to the result rest.

Run modes: enumerate every (x, rest) split; recover rest given an
element; recover an element given the rest; (and ground/all combinations).

6 new tests, 411/411 cumulative.

lib/minikanren/relations.sx

@@ -82,6 +82,14 @@
       (appendo front-and-s back l)
       (appendo front s front-and-s))))
 
+(define
+  selecto
+  (fn
+    (x rest l)
+    (conde
+      ((conso x rest l))
+      ((fresh (a d r) (conso a d l) (conso a r rest) (selecto x r d))))))
+
 (define
   lengtho
   (fn

lib/minikanren/tests/selecto.sx

@@ -0,0 +1,46 @@
+;; lib/minikanren/tests/selecto.sx — choose an element + rest of list.
+
+(mk-test
+  "selecto-enumerate"
+  (run*
+    q
+    (fresh
+      (x r)
+      (selecto x r (list 1 2 3))
+      (== q (list x r))))
+  (list
+    (list 1 (list 2 3))
+    (list 2 (list 1 3))
+    (list 3 (list 1 2))))
+
+(mk-test
+  "selecto-find-rest"
+  (run* q (selecto 2 q (list 1 2 3)))
+  (list (list 1 3)))
+
+(mk-test
+  "selecto-find-element"
+  (run*
+    q
+    (selecto
+      q
+      (list 1 3)
+      (list 1 2 3)))
+  (list 2))
+
+(mk-test
+  "selecto-element-not-present-fails"
+  (run* q (selecto 99 q (list 1 2 3)))
+  (list))
+
+(mk-test
+  "selecto-empty-list-fails"
+  (run* q (selecto q (list) (list)))
+  (list))
+
+(mk-test
+  "selecto-singleton"
+  (run* q (fresh (x r) (selecto x r (list :only)) (== q (list x r))))
+  (list (list :only (list))))
+
+(mk-tests-run!)