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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 convert_string(self, rest):
 """Convert a reST string to an HTML string. """

        try:
            html = publish_string(rest, writer_name='html')
        except SystemExit as e:
            err_exit('HTML conversion failed with error: %s' % e.code)
        else:
            if sys.version_info[0] >= 3:
                return html.decode('utf-8')
            return html