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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 transfer_assets(self, asset_id, transfer_spec, btc_change_script, fees):
 """ Creates a transaction for sending an asset. :param bytes asset_id: The ID of the asset being sent. :param TransferParameters transfer_spec: The parameters of the asset being transferred. :param bytes btc_change_script: The script where to send bitcoin change, if any. :param int fees: The fees to include in the transaction. :return: The resulting unsigned transaction. :rtype: CTransaction """

        return self.transfer(
            [(asset_id, transfer_spec)],
            TransferParameters(transfer_spec.unspent_outputs, None, btc_change_script, 0),
            fees)