def position_changed(previous)
      add = previous.object ? previous.object.padded_length : 0
      next_at = previous.at + add
      next_pos = Position.get(@object)
      next_pos.set(next_at)
    end
def position_changed(previous)
      add = previous.object ? previous.object.padded_length : 0
      next_at = previous.at + add
      next_pos = Position.get(@object)
      next_pos.set(next_at)
    end
when the argument changes position, we update the next objects
 position to reflect that change