code: @Override
      public Calendar declineCounter(Calendar counter) {
          Calendar declineCounter = transform(Method.DECLINE_COUNTER, counter);
          declineCounter.validate();
          return declineCounter;
      }
docs: <pre>
  3.4.8.  DECLINECOUNTER
  
  The "DECLINECOUNTER" method in a "VTODO" calendar component is used
  by an "Organizer" of the "VTODO" calendar component to reject a
  counter proposal offered by one of the "Attendees".  The "Organizer"
  sends the message to the "Attendee" that sent the "COUNTER" method to
  the "Organizer".
  </pre>