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/relations.sx

@@ -58,6 +58,14 @@
       ((nullo l))
       ((fresh (a d) (conso a d l) (nafc (== a x)) (not-membero x d))))))
 
+(define
+  subseto
+  (fn
+    (l1 l2)
+    (conde
+      ((nullo l1))
+      ((fresh (a d) (conso a d l1) (membero a l2) (subseto d l2))))))
+
 (define
   reverseo
   (fn
@@ -76,8 +84,8 @@
 
 (define rev-2o (fn (l result) (rev-acco l (list) result)))
 
-(define palindromeo (fn (l) (fresh (rev) (reverseo l rev) (== l rev))))
 
+(define palindromeo (fn (l) (fresh (rev) (reverseo l rev) (== l rev))))
 
 (define prefixo (fn (p l) (fresh (rest) (appendo p rest l))))