mk: flatteno — nested list flattener

Three conde clauses: nullo tree -> nullo flat; pairo tree -> recurse on
car & cdr, appendo their flattenings; otherwise tree must be a ground
non-list atom (nafc nullo + nafc pairo) and flat = (list tree).

Works on ground inputs of arbitrary nesting:
  (run* q (flatteno (list 1 (list 2 3) (list (list 4) 5)) q))
    -> ((1 2 3 4 5))

7 tests, 253/253 cumulative. Phase 4 list relations now complete.

lib/minikanren/relations.sx

@@ -81,3 +81,18 @@
     (conde
       ((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
+    (tree flat)
+    (conde
+      ((nullo tree) (nullo flat))
+      ((pairo tree)
+        (fresh
+          (h t hf tf)
+          (conso h t tree)
+          (flatteno h hf)
+          (flatteno t tf)
+          (appendo hf tf flat)))
+      ((nafc (nullo tree)) (nafc (pairo tree)) (== flat (list tree))))))

lib/minikanren/tests/flatteno.sx

@@ -0,0 +1,42 @@
+(mk-test "flatteno-empty" (run* q (flatteno (list) q)) (list (list)))
+
+(mk-test
+  "flatteno-atom"
+  (run* q (flatteno 5 q))
+  (list (list 5)))
+
+(mk-test
+  "flatteno-flat-list"
+  (run* q (flatteno (list 1 2 3) q))
+  (list (list 1 2 3)))
+
+(mk-test
+  "flatteno-singleton"
+  (run* q (flatteno (list 1) q))
+  (list (list 1)))
+
+(mk-test
+  "flatteno-nested-once"
+  (run*
+    q
+    (flatteno (list 1 (list 2 3) 4) q))
+  (list (list 1 2 3 4)))
+
+(mk-test
+  "flatteno-nested-twice"
+  (run*
+    q
+    (flatteno
+      (list
+        1
+        (list 2 (list 3 4))
+        5)
+      q))
+  (list (list 1 2 3 4 5)))
+
+(mk-test
+  "flatteno-keywords"
+  (run* q (flatteno (list :a (list :b :c) :d) q))
+  (list (list :a :b :c :d)))
+
+(mk-tests-run!)