def position_changed(position)
      instruction = position.object
      return unless instruction.is_a?(Label)
      instruction.address.set_value(position.at)
    end
def position_changed(position)
      instruction = position.object
      return unless instruction.is_a?(Label)
      instruction.address.set_value(position.at)
    end
update label positions. All else done in position_changing