mk: tako + dropo — Peano-indexed prefix and suffix

(tako n l prefix) — prefix is the first n elements of l.
(dropo n l suffix) — suffix is l after dropping the first n.

Both use a Peano natural for the count. Round-trip holds:
  (tako n l) ⊕ (dropo n l) = l   (verified by an end-to-end test)

9 new tests, 368/368 cumulative.

lib/minikanren/tests/take-drop.sx

@@ -0,0 +1,92 @@
+;; lib/minikanren/tests/take-drop.sx — Peano-indexed prefix/suffix.
+
+(define
+  mk-nat
+  (fn (n) (if (= n 0) :z (list :s (mk-nat (- n 1))))))
+
+;; --- tako ---
+
+(mk-test
+  "tako-zero"
+  (run*
+    q
+    (tako (mk-nat 0) (list 1 2 3) q))
+  (list (list)))
+
+(mk-test
+  "tako-two"
+  (run*
+    q
+    (tako
+      (mk-nat 2)
+      (list 1 2 3 4 5)
+      q))
+  (list (list 1 2)))
+
+(mk-test
+  "tako-all"
+  (run*
+    q
+    (tako (mk-nat 3) (list 1 2 3) q))
+  (list (list 1 2 3)))
+
+(mk-test
+  "tako-too-many"
+  (run*
+    q
+    (tako (mk-nat 5) (list 1 2 3) q))
+  (list))
+
+;; --- dropo ---
+
+(mk-test
+  "dropo-zero"
+  (run*
+    q
+    (dropo (mk-nat 0) (list 1 2 3) q))
+  (list (list 1 2 3)))
+
+(mk-test
+  "dropo-two"
+  (run*
+    q
+    (dropo
+      (mk-nat 2)
+      (list 1 2 3 4 5)
+      q))
+  (list (list 3 4 5)))
+
+(mk-test
+  "dropo-all"
+  (run*
+    q
+    (dropo (mk-nat 3) (list 1 2 3) q))
+  (list (list)))
+
+(mk-test
+  "dropo-too-many"
+  (run*
+    q
+    (dropo (mk-nat 5) (list 1 2 3) q))
+  (list))
+
+;; --- take + drop round-trip ---
+
+(mk-test
+  "tako-dropo-roundtrip"
+  (run*
+    q
+    (fresh
+      (p s)
+      (tako
+        (mk-nat 2)
+        (list 1 2 3 4 5)
+        p)
+      (dropo
+        (mk-nat 2)
+        (list 1 2 3 4 5)
+        s)
+      (appendo p s q)))
+  (list (list 1 2 3 4 5)))
+
+(mk-tests-run!)