    function traderApproveCurrentExchangeContract(uint _exchangeContractVersion) external {
        require (_exchangeContractVersion > 1, "First version doesn't need approval.");
        require (_exchangeContractVersion == exchangeContractVersion, "Can only approve the latest version.");
        traderApprovals[msg.sender] = _exchangeContractVersion;
        emit TraderApproveContract(msg.sender, _exchangeContractVersion);
    }