anchor.diff:  
   
   (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
  
positive.code: 
  
  (defun queue-peek (queue)
    (with-cas-retry
      (let ((head (queue-head queue)))
        (when head
  
negative.code: 
  
  (defun queue-push (queue item)
    "Add ITEM to the tail of QUEUE, returning as values ITEM and QUEUE."
    (symbol-macrolet
        ((head (queue-head queue))
         (tail (queue-tail queue)))
  