mk: btree-walko — binary-tree walker via matche

Demo of matche dispatch + conde + recursion for tree traversal:
  (matche tree
    ((:leaf x)   (== v x))
    ((:node l r) (conde ((btree-walko l v)) ((btree-walko r v)))))

Test tree ((1 2) (3 (4 5))) yields all 5 leaves under run*. Also tests
membership (run 1) and absence.

4 new tests, 441/441 cumulative.

lib/minikanren/tests/btree-walko.sx

@@ -0,0 +1,54 @@
+;; lib/minikanren/tests/btree-walko.sx — walk a leaves-of-binary-tree relation
+;; using matche dispatch on (:leaf v) and (:node left right) patterns.
+
+(define
+  btree-walko
+  (fn
+    (tree v)
+    (matche
+      tree
+      ((:leaf x) (== v x))
+      ((:node l r) (conde ((btree-walko l v)) ((btree-walko r v)))))))
+
+;; A small test tree: ((1 2) (3 (4 5))).
+(define
+  test-btree
+  (list
+    :node (list :node (list :leaf 1) (list :leaf 2))
+    (list
+      :node (list :leaf 3)
+      (list :node (list :leaf 4) (list :leaf 5)))))
+
+(mk-test
+  "btree-walko-enumerates-all-leaves"
+  (let
+    ((leaves (run* q (btree-walko test-btree q))))
+    (and
+      (= (len leaves) 5)
+      (and
+        (some (fn (l) (= l 1)) leaves)
+        (and
+          (some (fn (l) (= l 2)) leaves)
+          (and
+            (some (fn (l) (= l 3)) leaves)
+            (and
+              (some (fn (l) (= l 4)) leaves)
+              (some (fn (l) (= l 5)) leaves)))))))
+  true)
+
+(mk-test
+  "btree-walko-find-3-membership"
+  (run 1 q (btree-walko test-btree 3))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "btree-walko-find-99-not-present"
+  (run* q (btree-walko test-btree 99))
+  (list))
+
+(mk-test
+  "btree-walko-leaf-only"
+  (run* q (btree-walko (list :leaf 42) q))
+  (list 42))
+
+(mk-tests-run!)

plans/minikanren-on-sx.md

@@ -173,6 +173,7 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **btree-walko (matche showcase)**: walks a binary tree (:leaf v) | (:node l r) and emits each leaf value via conde. Demonstrates matche dispatch on tagged-list patterns, recursion through both branches via conde, and run* enumerating all 5 leaves of a small tree. 4 new tests, 441/441 cumulative.
 - **2026-05-08** — **swap-firsto**: swap the first two elements of a list. Built via four conso constraints. Self-inverse on length-2+ lists; runs forward and backward. 6 new tests, 437/437 cumulative.
 - **2026-05-08** — **pairlisto**: relational zip — pairs is the zipped list of (l1[i] l2[i]). Runs forward, recovers l1 given l2 and pairs, recovers l2 given l1 and pairs. 5 new tests, 431/431 cumulative.
 - **2026-05-08** — **iterate-no**: relational iterated application. Applies a 2-arg relation n times (Peano n) starting from x to produce result. Generalises succ-iteration / list-cons-iteration / etc. 4 new tests, 426/426 cumulative.