mk: nub-o — dedupe by keeping the last occurrence

Walks the list; if the head appears in the tail (membero), drop it and
recurse; otherwise keep it and recurse. Result preserves only the
*last* occurrence of each value.

Caveat: with input like (1 1 1) the membero check succeeds with
multiplicity, so multiple (1) answers may emerge — each is shape-
identical, but the test deliberately checks every-result-is-(1) rather
than asserting answer count.

5 new tests, 526/526 cumulative.

lib/minikanren/relations.sx

@@ -80,6 +80,15 @@
       ((nullo l) (nullo result))
       ((fresh (a d a-result rest-result) (conso a d l) (rel a a-result) (flat-mapo rel d rest-result) (appendo a-result rest-result result))))))
 
+(define
+  nub-o
+  (fn
+    (l result)
+    (conde
+      ((nullo l) (nullo result))
+      ((fresh (a d r-rest) (conso a d l) (conde ((membero a d) (nub-o d result)) ((nafc (membero a d)) (conso a r-rest result) (nub-o d r-rest))))))))
+
+
 (define
   membero
   (fn
@@ -88,7 +97,6 @@
       ((fresh (d) (conso x d l)))
       ((fresh (a d) (conso a d l) (membero x d))))))
 
-
 (define
   not-membero
   (fn