def unschedule_next
      clock, (obj, relative_time) = next_with_clock
      if obj
        clock.unschedule(obj)
        [obj, relative_time]
      end
    end
def unschedule_next
      clock, (obj, relative_time) = next_with_clock
      if obj
        clock.unschedule(obj)
        [obj, relative_time]
      end
    end
unschedules and returns the next object as [obj, relative_time],
 where relative_time is the offset from now in parent's time units