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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 set_xml_output(self, xml_file):
 """ Tell ligolw_sqlite to dump the contents of the database to a file. """

    if self.get_database() is None:
      raise ValueError, "no database specified"
    self.add_file_opt('extract', xml_file)
    self.__xml_output = xml_file