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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 column_names(self):
 """ Returns the column names. Returns ------- out : list[string] Column names of the SFrame. """

        if self._is_vertex_frame():
            return self.__graph__.__proxy__.get_vertex_fields()
        elif self._is_edge_frame():
            return self.__graph__.__proxy__.get_edge_fields()