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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(self):
 """ Accesses the parse tree based on the S-expression parse string in the XML :getter: Returns the NLTK parse tree :type: nltk.Tree """

        if self.parse_string is not None and self._parse is None:
            self._parse = Tree.parse(self._parse_string)
        return self._parse