    bytes memory payload = abi.encode(VERSION, msg.sender, amount, nonce++);
  function _payGasAndCallCotract(
    string calldata destinationChain,
    string memory destContract,
    bytes memory payload
  ) private {
    GAS_RECEIVER.payNativeGasForContractCall{value: msg.value}(
      address(this),
      destinationChain,
      destContract,
      payload,
      msg.sender
    );

    // Send all information to AxelarGateway contract.
    AXELAR_GATEWAY.callContract(destinationChain, destContract, payload);
  }
  function _execute(
    string calldata srcChain,
    string calldata srcAddr,
    bytes calldata payload
  ) internal override whenNotPaused {
  
    (bytes32 version, address srcSender, uint256 amt, uint256 nonce) = abi
      .decode(payload, (bytes32, address, uint256, uint256));

    if (version != VERSION) {
      revert InvalidVersion();
    }
    if (chainToApprovedSender[srcChain] == bytes32(0)) {
      revert ChainNotSupported();
    }
    // each chain have only one approved sender that is the source bridge contract.
    if (chainToApprovedSender[srcChain] != keccak256(abi.encode(srcAddr))) {
      revert SourceNotSupported();
    }
    if (isSpentNonce[chainToApprovedSender[srcChain]][nonce]) {
      revert NonceSpent();
    }

    isSpentNonce[chainToApprovedSender[srcChain]][nonce] = true;

    // same payload would have the same txhash
    bytes32 txnHash = keccak256(payload);
    txnHashToTransaction[txnHash] = Transaction(srcSender, amt);
    _attachThreshold(amt, txnHash, srcChain);
    _approve(txnHash);
    _mintIfThresholdMet(txnHash);
    emit MessageReceived(srcChain, srcSender, amt, nonce);
  }
