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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 _ParseFileVersion(file_version):
 """Convert the string file_version in event.proto into a float. Args: file_version: String file_version from event.proto Returns: Version number as a float. """

  tokens = file_version.split('brain.Event:')
  try:
    return float(tokens[-1])
  except ValueError:
    ## This should never happen according to the definition of file_version
    ## specified in event.proto.
    logger.warn(
        ('Invalid event.proto file_version. Defaulting to use of '
         'out-of-order event.step logic for purging expired events.'))
    return -1