new_contents: from stellar_sdk.account import Account
  from stellar_sdk.keypair import Keypair
  from stellar_sdk.network import Network
  from stellar_sdk.transaction_builder import TransactionBuilder
  
  inner_keypair = Keypair.from_secret(
      "SBKTIFHJSS3JJWEZO2W74DZSA45WZU56LOL3AY7GAW63BXPEJQFYV53E"
  )
  inner_source = Account(inner_keypair.public_key, 7)
  destination = "GDQERENWDDSQZS7R7WKHZI3BSOYMV3FSWR7TFUYFTKQ447PIX6NREOJM"
  amount = "2000"
  inner_tx = (
      TransactionBuilder(inner_source, Network.TESTNET_NETWORK_PASSPHRASE, 200, v1=True)
      .append_payment_op(destination=destination, amount=amount, asset_code="XLM")
      .build()
  )
  inner_tx.sign(inner_keypair)
  fee_source = Keypair.from_secret(
      "SB7ZMPZB3YMMK5CUWENXVLZWBK4KYX4YU5JBXQNZSK2DP2Q7V3LVTO5V"
  )
  base_fee = 200
  fee_bump_tx = TransactionBuilder.build_fee_bump_transaction(
      fee_source.public_key, base_fee, inner_tx, Network.TESTNET_NETWORK_PASSPHRASE,
  )
  fee_bump_tx.sign(fee_source)
  
subject: build fee bump transaction.