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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 btc_asset_swap(self, btc_transfer_spec, asset_id, asset_transfer_spec, fees):
 """ Creates a transaction for swapping assets for bitcoins. :param TransferParameters btc_transfer_spec: The parameters of the bitcoins being transferred. :param bytes asset_id: The ID of the asset being sent. :param TransferParameters asset_transfer_spec: The parameters of the asset being transferred. :param int fees: The fees to include in the transaction. :return: The resulting unsigned transaction. :rtype: CTransaction """

        return self.transfer([(asset_id, asset_transfer_spec)], btc_transfer_spec, fees)