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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 head(self):
 """ The token serving as the "head" of the mention :getter: the token corresponding to the head :type: corenlp_xml.document.Token """

        if self._head is None:
            self._head = self.sentence.tokens[self._head_id-1]
        return self._head