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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def GetVolumeIdentifiers(self, volume_system):
 """Retrieves the volume identifiers. Args: volume_system (VolumeSystem):
 volume system. Returns: list[str]: sorted volume identifiers. """

    volume_identifiers = []
    for volume in volume_system.volumes:
      volume_identifier = getattr(volume, 'identifier', None)
      if volume_identifier:
        volume_identifiers.append(volume_identifier)

    return sorted(volume_identifiers)