here's an append-o for you.

it's kind of simple, but also subtle, because it can get minikanren to
not find all the things.

scm/logic.scm

@@ -21,3 +21,12 @@
(define cons-o
  (lambda (x l r)
    (== r (cons x l))))
(define append-o
  (lambda (l s r)
    (conde
     ((== l '()) (== s r))
     ((fresh (a d res)
             (== `(,a . ,d) l)
             (== `(,a . ,res) r)
             (append-o d s res))))))