mk: subseto — every element of l1 is in l2

Recursive: empty l1 trivially holds; otherwise the head is in l2 (via
membero) and the tail is a subset. Duplicates in l1 are allowed since
each is independently checked.

7 new tests, 456/456 cumulative.

lib/minikanren/tests/subseto.sx

@@ -0,0 +1,62 @@
+;; lib/minikanren/tests/subseto.sx — every element of l1 is in l2.
+
+(mk-test
+  "subseto-empty"
+  (run* q (subseto (list) (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "subseto-singleton-yes"
+  (run*
+    q
+    (subseto (list 2) (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "subseto-singleton-no"
+  (run*
+    q
+    (subseto (list 99) (list 1 2 3)))
+  (list))
+
+(mk-test
+  "subseto-multi-yes"
+  (run
+    1
+    q
+    (subseto
+      (list 1 3)
+      (list 1 2 3 4)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "subseto-multi-no"
+  (run*
+    q
+    (subseto
+      (list 1 99)
+      (list 1 2 3)))
+  (list))
+
+(mk-test
+  "subseto-equal-sets"
+  (run
+    1
+    q
+    (subseto
+      (list 1 2 3)
+      (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+;; allow duplicates in l1 — each just needs membership in l2.
+(mk-test
+  "subseto-duplicates-allowed"
+  (run
+    1
+    q
+    (subseto
+      (list 1 1 2)
+      (list 1 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-tests-run!)