GUSD合约关系

ERC20Proxy通过继承ERC20ImplUpgradeable，可以调用ERC20Impl合约。

ERC20Impl里有两个合约对象，一个是ERC20Proxy，一个是ERC20Store，ERC20Proxy只是传递一些事件，如：

        erc20Proxy.emitApproval(_sender, _spender, _value);

而ERC20Store是真正的数据存储。

ERC20Proxy和ERC20Impl是互相引用，主要是ERC20Proxy调用ERC20Impl，而ERC20Impl只是事件通知时会调用ERC20Proxy。

GUSD合约部署

Custodian只用在以下几个函数用来判断：

    function confirmCustodianChange(bytes32 _lockId) public onlyCustodian {
        custodian = getCustodianChangeReq(_lockId);

        delete custodianChangeReqs[_lockId];

        emit CustodianChangeConfirmed(_lockId, custodian);
    }

    function confirmImplChange(bytes32 _lockId) public onlyCustodian {
        erc20Impl = getImplChangeReq(_lockId);

        delete implChangeReqs[_lockId];

        emit ImplChangeConfirmed(_lockId, address(erc20Impl));
    }

    function confirmPrint(bytes32 _lockId) public onlyCustodian {
        PendingPrint storage print = pendingPrintMap[_lockId];

        // reject ‘null’ results from the map lookup
        // this can only be the case if an unknown `_lockId` is received
        address receiver = print.receiver;
        require (receiver != address(0));
        uint256 value = print.value;

        delete pendingPrintMap[_lockId];

        uint256 supply = erc20Store.totalSupply();
        uint256 newSupply = supply + value;
        if (newSupply >= supply) {
          erc20Store.setTotalSupply(newSupply);
          erc20Store.addBalance(receiver, value);

          emit PrintingConfirmed(_lockId, receiver, value);
          erc20Proxy.emitTransfer(address(0), receiver, value);
        }
    }