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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 arg_dict(self):
 """Get dictionary representation of argument arrrays. Returns ------- arg_dict : dict of str to NDArray The dictionary that maps the names of arguments to NDArrays. Raises ------ ValueError : if there are duplicated names in the arguments. """

        if self._arg_dict is None:
            self._arg_dict = Executor._get_dict(
                self._symbol.list_arguments(), self.arg_arrays)
        return self._arg_dict