anchor.diff:    "Get a list of the names of all pdefs playing on CLOCK."
     (loop :for i :in (slot-value clock 'tasks)
        :collect (slot-value (slot-value i 'item) 'key)))
  +(defmethod end ((list list))
  +  (mapcar 'end list))
   
   (defmethod playing-p ((task task) &optional (clock *clock*))
     (position task (slot-value clock 'tasks)))
  
positive.code: (defmethod stop ((task task))
    (clock-remove task))
  
  (defmethod end ((item pdef))
    (with-slots (key) item
      (if (pdef-ref-get key :task)
  
negative.code: 
  (defmethod playing-p ((key symbol) &optional (clock *clock*))
    (position key (pdefs-playing clock)))
  
  (defmethod playing-p ((list list) &optional (clock *clock*))
    (mapcar (lambda (item) (playing-p item clock)) list))
  