function Upgradable(address _prevVersion) public {
        if (_prevVersion != address(0)) {
            require(msg.sender == Ownable(_prevVersion).owner());
            upgradableState.isUpgrading = true;
            upgradableState.prevVersion = _prevVersion;
            IUpgradable(_prevVersion).startUpgrade();
        } else {
            Initialized(_prevVersion);
        }
    }