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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 parse_string(self):
 """ Accesses the S-Expression parse string stored on the XML document :getter: Returns the parse string :type: str """

        if self._parse_string is None:
            parse_text = self._element.xpath('parse/text()')
            if len(parse_text) > 0:
                self._parse_string = parse_text[0]
        return self._parse_string