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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 tojson(self):
 """Saves symbol to a JSON string. See Also -------- symbol.load_json : Used to load symbol from JSON string. """

        json_str = ctypes.c_char_p()
        check_call(_LIB.MXSymbolSaveToJSON(self.handle, ctypes.byref(json_str)))
        return py_str(json_str.value)