chickadee » skiplists » skiplist-insert!

command ((oldcount newcount (lambda (sls . items) (skiplist-count sls)))
         (oldfound newfound (lambda (sls . items)
                              (skiplist-search! sls (car items))
                              (skiplist-found sls))))
requires (and (skiplist? sls)
              ((list-of? (skiplist-item? sls)) (cons item items)))
ensures  (and (fx>= newcount oldcount) (member item newfound))