  function payDai(uint256 _daiAmount) external {
    require(iERC20Token(daiToken).transferFrom(msg.sender, address(this), _daiAmount), "failed to transfer dai");
    holdoverDaiBalance = safeAdd(holdoverDaiBalance, _daiAmount);
    totalDaiReceived = safeAdd(totalDaiReceived, _daiAmount);
  }