    function acceptNextExchangeContract() external {
        require (msg.sender == nextExchangeContract, "Can only accept preapproved exchange contract.");
        exchangeContract = nextExchangeContract;
        prevExchangeContracts[nextExchangeContract] = true;
        exchangeContractVersion++;
        emit UpgradeExchangeContract(exchangeContract, exchangeContractVersion);
    }