anchor.diff:  (defmethod stop ((task task))
     (clock-remove task))
   
  +(defmethod stop ((list list))
  +  (mapcar 'stop list))
  +
   (defmethod end ((item pdef))
     (with-slots (key) item
       (if (pdef-ref-get key :task)
  
positive.code: 
  (defmethod launch ((symbol symbol))
    (launch (pdef symbol)))
  
  (defmethod stop ((pdef pdef))
    (with-slots (key) pdef
  
negative.code: (defmethod play ((symbol symbol))
    (play (pdef symbol)))
  
  (defmethod play ((list list))
    (mapcar 'play list))
  
  (defmethod launch ((event event))
    (play event))
  
  