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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 _WriteHeader(self, output_writer):
 """Writes a header. Args: output_writer (OutputWriter):
 output writer. """

    header_string = ''
    if self._title:
      header_string = ' {0:s} '.format(self._title)

    header_string = self._HEADER_FORMAT_STRING.format(header_string)
    output_writer.Write(header_string)