code: public synchronized long nextFetch() {
  		assert visitStates.peek() != null;
  		return Math.max(nextFetch, visitStates.peek().nextFetch);
  	}
docs: Returns the minimum time at which some URL in some {@link VisitState} of the visit-state queue
  of this workbench entry can be accessed.
  It is computed by maximizing {@link #nextFetch} with {@link VisitState#nextFetch} of the
  top element of the visit-state queue.
  
  <p>Note that this method will access blindly the top element, which might cause a {@link NullPointerException}
  if the visit-state queue is empty. Thus, great care must be exercised in placing on the workbench
  entries with a nonempty visit-state queue only.
  
  @return the minimum time at which some URL in some {@link VisitState} of this entry can be accessed.