From fc14a8063b41eaef642d2ee43b12af05323b3747 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 8 May 2026 11:40:28 +0000
Subject: [PATCH] =?UTF-8?q?mk:=20prefixo=20+=20suffixo=20=E2=80=94=20appen?=
 =?UTF-8?q?do-derived=20sublist=20relations?=
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Two-line definitions over appendo:
  (prefixo p l) ≡ ∃rest. (appendo p rest l)
  (suffixo s l) ≡ ∃front. (appendo front s l)

Both enumerate all prefixes/suffixes when called with a fresh first
arg, and serve as decision relations when called with both grounded.

9 new tests, 398/398 cumulative.
---
 lib/minikanren/relations.sx           |  6 ++-
 lib/minikanren/tests/prefix-suffix.sx | 76 +++++++++++++++++++++++++++
 plans/minikanren-on-sx.md             |  1 +
 3 files changed, 82 insertions(+), 1 deletion(-)
 create mode 100644 lib/minikanren/tests/prefix-suffix.sx

diff --git a/lib/minikanren/relations.sx b/lib/minikanren/relations.sx
index 2475125a..b86b5e3b 100644
--- a/lib/minikanren/relations.sx
+++ b/lib/minikanren/relations.sx
@@ -68,6 +68,11 @@
 
 (define palindromeo (fn (l) (fresh (rev) (reverseo l rev) (== l rev))))
 
+(define prefixo (fn (p l) (fresh (rest) (appendo p rest l))))
+
+(define suffixo (fn (s l) (fresh (front) (appendo front s l))))
+
+
 (define
   lengtho
   (fn
@@ -84,7 +89,6 @@
       ((conso a l p))
       ((fresh (h t pt) (conso h t l) (conso h pt p) (inserto a t pt))))))
 
-
 (define
   permuteo
   (fn
diff --git a/lib/minikanren/tests/prefix-suffix.sx b/lib/minikanren/tests/prefix-suffix.sx
new file mode 100644
index 00000000..7237ef1d
--- /dev/null
+++ b/lib/minikanren/tests/prefix-suffix.sx
@@ -0,0 +1,76 @@
+;; lib/minikanren/tests/prefix-suffix.sx — appendo-derived sublist relations.
+
+(mk-test
+  "prefixo-empty"
+  (run* q (prefixo (list) (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "prefixo-full"
+  (run*
+    q
+    (prefixo
+      (list 1 2 3)
+      (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "prefixo-partial"
+  (run*
+    q
+    (prefixo
+      (list 1 2)
+      (list 1 2 3 4)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "prefixo-mismatch-fails"
+  (run*
+    q
+    (prefixo
+      (list 1 3)
+      (list 1 2 3)))
+  (list))
+
+(mk-test
+  "prefixo-enumerates-all"
+  (run* q (prefixo q (list 1 2 3)))
+  (list
+    (list)
+    (list 1)
+    (list 1 2)
+    (list 1 2 3)))
+
+(mk-test
+  "suffixo-empty"
+  (run* q (suffixo (list) (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "suffixo-full"
+  (run*
+    q
+    (suffixo
+      (list 1 2 3)
+      (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "suffixo-partial"
+  (run*
+    q
+    (suffixo
+      (list 2 3)
+      (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "suffixo-enumerates-all"
+  (run* q (suffixo q (list 1 2 3)))
+  (list
+    (list 1 2 3)
+    (list 2 3)
+    (list 3)
+    (list)))
+
+(mk-tests-run!)
diff --git a/plans/minikanren-on-sx.md b/plans/minikanren-on-sx.md
index cb59ed9b..a12b67b3 100644
--- a/plans/minikanren-on-sx.md
+++ b/plans/minikanren-on-sx.md
@@ -173,6 +173,7 @@ _(none yet)_
 
 _Newest first._
 
+- **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".
   Uses `nafc + ==` per element (the same skeleton all-distincto uses).