anchor.diff:  
   
   (defun queue-empty-p (queue)
  +  "Return non-nil when QUEUE is \(probably\) empty."
     (with-cas-retry
       (let ((head (queue-head queue)))
         (or (null head)
  
positive.code:           (multiple-value-bind (item empty-p)
                (queue-dequeue queue)
              (unless empty-p
                #-deploy (assert (not (eq +empty+ item)))
                (return (values item empty-p))))
            (when (and end-time
                       (>= (get-internal-real-time) end-time))
  
negative.code: 
  
  (defun queue-count (q)
    "Return the approximate number of items in the queue."
    (max 0 (- (#+sbcl aref
               #-sbcl svref (queue-enqueued-count q) 0)
              (#+sbcl aref
  