code: def btc_asset_swap(self, btc_transfer_spec, asset_id, asset_transfer_spec, fees):
          
          return self.transfer([(asset_id, asset_transfer_spec)], btc_transfer_spec, fees)
docs: 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