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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 NetworkFee(self):
 """ Get the network fee. Returns: Fixed8: """

        if self._network_fee is None:

            input = Fixed8(0)

            for coin_ref in self.References.values():
                if coin_ref.AssetId == GetBlockchain().SystemCoin().Hash:
                    input = input + coin_ref.Value

            output = Fixed8(0)

            for tx_output in self.outputs:
                if tx_output.AssetId == GetBlockchain().SystemCoin().Hash:
                    output = output + tx_output.Value

            self._network_fee = input - output - self.SystemFee()

        #            logger.info("Determined network fee to be %s " % (self.__network_fee.value))

        return self._network_fee