From e202c81a0d4817da4e2a0f59a0fb5eeb1c029b58 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 8 May 2026 11:45:31 +0000
Subject: [PATCH] =?UTF-8?q?mk:=20subo=20=E2=80=94=20contiguous=20sublist?=
 =?UTF-8?q?=20relation?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Composes two appendos: l = front ++ s ++ back, equivalently
  (appendo front-and-s back l) and (appendo front s front-and-s).

Goal order matters: doing the (appendo ground:l) split first makes the
search finitary; the second appendo is then deterministic given
front-and-s and ground s. Reversing the order causes divergence on
failing inputs (the front search becomes unbounded).

7 new tests, 405/405 cumulative.
---
 lib/minikanren/relations.sx  |  9 ++++++
 lib/minikanren/tests/subo.sx | 60 ++++++++++++++++++++++++++++++++++++
 plans/minikanren-on-sx.md    |  1 +
 3 files changed, 70 insertions(+)
 create mode 100644 lib/minikanren/tests/subo.sx

diff --git a/lib/minikanren/relations.sx b/lib/minikanren/relations.sx
index b86b5e3b..4b9c61c2 100644
--- a/lib/minikanren/relations.sx
+++ b/lib/minikanren/relations.sx
@@ -73,6 +73,15 @@
 (define suffixo (fn (s l) (fresh (front) (appendo front s l))))
 
 
+(define
+  subo
+  (fn
+    (s l)
+    (fresh
+      (front-and-s back front)
+      (appendo front-and-s back l)
+      (appendo front s front-and-s))))
+
 (define
   lengtho
   (fn
diff --git a/lib/minikanren/tests/subo.sx b/lib/minikanren/tests/subo.sx
new file mode 100644
index 00000000..a48c645a
--- /dev/null
+++ b/lib/minikanren/tests/subo.sx
@@ -0,0 +1,60 @@
+;; lib/minikanren/tests/subo.sx — contiguous-sublist relation.
+
+(mk-test
+  "subo-simple-found"
+  (run*
+    q
+    (subo
+      (list 2 3)
+      (list 1 2 3 4)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "subo-not-contiguous-fails"
+  (run*
+    q
+    (subo
+      (list 2 4)
+      (list 1 2 3 4)))
+  (list))
+
+(mk-test
+  "subo-full-list-found"
+  (run*
+    q
+    (subo
+      (list 1 2 3)
+      (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "subo-empty-list-found"
+  (let
+    ((res (run* q (subo (list) (list 1 2 3)))))
+    (= (len res) 4))
+  true)
+
+(mk-test
+  "subo-prefix"
+  (run*
+    q
+    (subo
+      (list 1 2)
+      (list 1 2 3 4)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "subo-suffix"
+  (run*
+    q
+    (subo
+      (list 3 4)
+      (list 1 2 3 4)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "subo-strings"
+  (run* q (subo (list "b" "c") (list "a" "b" "c" "d")))
+  (list (make-symbol "_.0")))
+
+(mk-tests-run!)
diff --git a/plans/minikanren-on-sx.md b/plans/minikanren-on-sx.md
index a12b67b3..b2dcaa2e 100644
--- a/plans/minikanren-on-sx.md
+++ b/plans/minikanren-on-sx.md
@@ -173,6 +173,7 @@ _(none yet)_
 
 _Newest first._
 
+- **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.
 - **2026-05-08** — **not-membero**: relational "x is not a member of l".