mk: palindromeo — list reads same forwards/backwards

Two-line definition: a list is a palindrome iff it equals its reverse.
Direct composition of reverseo + ==.

7 new tests: empty / singleton / equal pair / unequal pair /
5-element-yes / 5-element-no / strings.

389/389 cumulative.

lib/minikanren/relations.sx

@@ -66,6 +66,8 @@
       ((nullo l) (nullo r))
       ((fresh (a d res-rev) (conso a d l) (reverseo d res-rev) (appendo res-rev (list a) r))))))
 
+(define palindromeo (fn (l) (fresh (rev) (reverseo l rev) (== l rev))))
+
 (define
   lengtho
   (fn
@@ -82,6 +84,7 @@
       ((conso a l p))
       ((fresh (h t pt) (conso h t l) (conso h pt p) (inserto a t pt))))))
 
+
 (define
   permuteo
   (fn
@@ -90,7 +93,6 @@
       ((nullo l) (nullo p))
       ((fresh (a d perm-d) (conso a d l) (permuteo d perm-d) (inserto a perm-d p))))))
 
-
 (define
   flatteno
   (fn

lib/minikanren/tests/palindromeo.sx

@@ -0,0 +1,44 @@
+;; lib/minikanren/tests/palindromeo.sx — palindromic list relation.
+
+(mk-test
+  "palindromeo-empty"
+  (run* q (palindromeo (list)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "palindromeo-singleton"
+  (run* q (palindromeo (list :a)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "palindromeo-pair-equal"
+  (run* q (palindromeo (list 1 1)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "palindromeo-pair-unequal-fails"
+  (run* q (palindromeo (list 1 2)))
+  (list))
+
+(mk-test
+  "palindromeo-five-yes"
+  (run*
+    q
+    (palindromeo
+      (list 1 2 3 2 1)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "palindromeo-five-no"
+  (run*
+    q
+    (palindromeo
+      (list 1 2 3 4 5)))
+  (list))
+
+(mk-test
+  "palindromeo-strings"
+  (run* q (palindromeo (list "a" "b" "a")))
+  (list (make-symbol "_.0")))
+
+(mk-tests-run!)

plans/minikanren-on-sx.md

@@ -173,6 +173,7 @@ _(none yet)_
 
 _Newest first._
 
+- **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).
   Useful as a constraint-style filter: `(membero x dom) (not-membero x