From 4d861575df8842b82ad152095575a07760f84194 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 8 May 2026 11:47:27 +0000
Subject: [PATCH] =?UTF-8?q?mk:=20selecto=20=E2=80=94=20choose=20element=20?=
 =?UTF-8?q?+=20rest=20of=20list?=
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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     |  8 ++++++
 lib/minikanren/tests/selecto.sx | 46 +++++++++++++++++++++++++++++++++
 plans/minikanren-on-sx.md       |  1 +
 3 files changed, 55 insertions(+)
 create mode 100644 lib/minikanren/tests/selecto.sx

diff --git a/lib/minikanren/relations.sx b/lib/minikanren/relations.sx
index 4b9c61c2..b2b837a8 100644
--- a/lib/minikanren/relations.sx
+++ b/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
diff --git a/lib/minikanren/tests/selecto.sx b/lib/minikanren/tests/selecto.sx
new file mode 100644
index 00000000..ba34a86b
--- /dev/null
+++ b/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!)
diff --git a/plans/minikanren-on-sx.md b/plans/minikanren-on-sx.md
index b2dcaa2e..b040fb45 100644
--- a/plans/minikanren-on-sx.md
+++ b/plans/minikanren-on-sx.md
@@ -173,6 +173,7 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **selecto**: classic miniKanren "choose an element + rest". Direct base (l = (x . rest)) plus skip-head recurse. Enumerates all (element, rest) splits in run*; runs forward, backward, mid-pipeline. 6 new tests, 411/411 cumulative.
 - **2026-05-08** — **subo (contiguous sublist)**: Two appendos chained — l = front ++ s ++ back. Goal order matters: appendo on the ground l first, so the search is finitary; then constrain front. 7 new tests, 405/405 cumulative.
 - **2026-05-08** — **prefixo + suffixo**: classic appendo-derived sublist relations. (prefixo p l) ≡ p ⊕ ? = l; (suffixo s l) ≡ ? ⊕ s = l. Both enumerate all prefixes/suffixes when given a fresh first arg. 9 new tests, 398/398 cumulative.
 - **2026-05-08** — **palindromeo**: 2-line definition (reverseo + ==). Succeeds when a list reads the same forwards and backwards. 7 new tests, 389/389 cumulative.