<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def device_changed(self, old_state, new_state):
 """Mount newly mountable devices."""

        # udisks2 sometimes adds empty devices and later updates them - which
        # makes is_external become true at a time later than device_added:
        if (self._mounter.is_addable(new_state)
                and not self._mounter.is_addable(old_state)
                and not self._mounter.is_removable(old_state)):
            self.auto_add(new_state)