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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 GetAPFSVolumeByPathSpec(self, path_spec):
 """Retrieves an APFS volume for a path specification. Args: path_spec (PathSpec):
 path specification. Returns: pyfsapfs.volume: an APFS volume or None if not available. """

    volume_index = apfs_helper.APFSContainerPathSpecGetVolumeIndex(path_spec)
    if volume_index is None:
      return None

    return self._fsapfs_container.get_volume(volume_index)