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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 GetSeverityString(self, severity):
 """Retrieves a string representation of the severity. Args: severity (int):
 severity. Returns: str: description of the event severity. """

    if 0 <= severity < len(self._SEVERITY):
      return self._SEVERITY[severity]
    return 'Unknown {0:d}'.format(severity)