code: @Override
      public Calendar counter(Calendar request) {
          Calendar counter = transform(Method.COUNTER, request);
          counter.validate();
          return counter;
      }
docs: <pre>
  
  3.4.7.  COUNTER
  
  The "COUNTER" method in a "VTODO" calendar component is used by an
  "Attendee" of an existing "VTODO" calendar component to submit to the
  "Organizer" a counter proposal for the "VTODO" calendar component.
  
  The counter proposal is an iCalendar object consisting of a "VTODO"
  calendar component that provides the complete description of the
  alternate "VTODO" calendar component.
  
  The "Organizer" rejects the counter proposal by sending the
  "Attendee" a "DECLINECOUNTER" method.  The "Organizer" accepts the
  counter proposal by rescheduling the to-do as described in
  Section 3.4.2.1, "REQUEST for Rescheduling a To-Do".  The
  "Organizer's" CUA SHOULD send a "REQUEST" message to all "Attendees"
  affected by any change triggered by an accepted "COUNTER".
  </pre>