uint256 actualFromAmount = LibAsset.getOwnBalance(fromAssetId);
(bool success, bytes memory res) = _swapData.callTo.call{ value: msg.value }(_swapData.callData);
if (!success) {
  string memory reason = LibUtil.getRevertMsg(res);
  revert(reason);
}
actualFromAmount -= LibAsset.getOwnBalance(fromAssetId);
require(fromAmount >= actualFromAmount, 'actual amount used more than specified');
// transfer excess back to user
if (actualFromAmount != fromAmount) {
  // transfer excess to user
  // difference calculation be unchecked since fromAmount > actualFromAmount
}
