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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 __add(self, dest, xpath):
 """ Adds a Xpath expression to the dest list :param dest: The destination list to add the Xpath :type dest: list :param xpath: The Xpath expression to add :type xpath: str """

        assert isinstance(xpath, string_types)
        dest.append(xpath)