anchor.diff:  
   
   (defun input-context-wait-test (stream)
  -  (declare (ignore stream))
     (let* ((queue (stream-input-buffer stream))
   	 (event (event-queue-peek queue)))
       (when event
  
positive.code: 		   (*input-wait-test* #'input-context-wait-test)
  		   (*input-wait-handler* #'input-context-event-handler))
  	       (return-from ,return-block ,form )))
  	 (cond ,@(mapcar #'(lambda (pointer-case)
  			     (destructuring-bind (case-type &body case-body)
  				 pointer-case
  
negative.code:     `(block ,return-block
         (multiple-value-bind ,vars
  	   (block ,context-block
  	     (let ((*input-context*
  		    (cons (cons (expand-presentation-type-abbreviation ,type)
                                  #'(lambda (object type event options)
  				    (return-from ,context-block
  				      (values object type event options))))
  			  ,(if override nil '*input-context*)))
  