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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 GetVolumeByIdentifier(self, volume_identifier):
 """Retrieves a specific volume based on the identifier. Args: volume_identifier (str):
 identifier of the volume within the volume system. Returns: Volume: a volume. """

    if not self._is_parsed:
      self._Parse()
      self._is_parsed = True

    return self._volumes[volume_identifier]