USDTのコントラクトを読む

# はじめに（Introduction）

実際に使われているトークン（ERC20）のコントラクトコードを読んでみようと思います。
有名なUSDTを見てみようと思います。

https://tether.to/en/

> 2014 年に開始された Tether トークン (USD₮) は、ステーブルコイン モデルの先駆者であり、最も広く取引されています。Tether トークンは、法定通貨の安定性とシンプルさ、そしてブロックチェーン テクノロジーの革新性を兼ね備えており、両方の長所を完璧に組み合わせています。

# USDT

USDTはEtherscanに`verification`されているので以下にSolidityのソースコードが公開されています。

https://etherscan.io/address/0xdac17f958d2ee523a2206206994597c13d831ec7#code

<details><summary>コード全文</summary>

```javascript
/**
 *Submitted for verification at Etherscan.io on 2017-11-28
 */

pragma solidity ^0.4.17;

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) {
            return 0;
        }
        uint256 c = a * b;
        assert(c / a == b);
        return c;
    }

    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        // assert(b > 0); // Solidity automatically throws when dividing by 0
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold
        return c;
    }

    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        assert(b <= a);
        return a - b;
    }

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        assert(c >= a);
        return c;
    }
}

/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {
    address public owner;

    /**
     * @dev The Ownable constructor sets the original `owner` of the contract to the sender
     * account.
     */
    function Ownable() public {
        owner = msg.sender;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(msg.sender == owner);
        _;
    }

    /**
     * @dev Allows the current owner to transfer control of the contract to a newOwner.
     * @param newOwner The address to transfer ownership to.
     */
    function transferOwnership(address newOwner) public onlyOwner {
        if (newOwner != address(0)) {
            owner = newOwner;
        }
    }
}

/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20Basic {
    uint public _totalSupply;

    function totalSupply() public constant returns (uint);

    function balanceOf(address who) public constant returns (uint);

    function transfer(address to, uint value) public;

    event Transfer(address indexed from, address indexed to, uint value);
}

/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20 is ERC20Basic {
    function allowance(
        address owner,
        address spender
    ) public constant returns (uint);

    function transferFrom(address from, address to, uint value) public;

    function approve(address spender, uint value) public;

    event Approval(address indexed owner, address indexed spender, uint value);
}

/**
 * @title Basic token
 * @dev Basic version of StandardToken, with no allowances.
 */
contract BasicToken is Ownable, ERC20Basic {
    using SafeMath for uint;

    mapping(address => uint) public balances;

    // additional variables for use if transaction fees ever became necessary
    uint public basisPointsRate = 0;
    uint public maximumFee = 0;

    /**
     * @dev Fix for the ERC20 short address attack.
     */
    modifier onlyPayloadSize(uint size) {
        require(!(msg.data.length < size + 4));
        _;
    }

    /**
     * @dev transfer token for a specified address
     * @param _to The address to transfer to.
     * @param _value The amount to be transferred.
     */
    function transfer(address _to, uint _value) public onlyPayloadSize(2 * 32) {
        uint fee = (_value.mul(basisPointsRate)).div(10000);
        if (fee > maximumFee) {
            fee = maximumFee;
        }
        uint sendAmount = _value.sub(fee);
        balances[msg.sender] = balances[msg.sender].sub(_value);
        balances[_to] = balances[_to].add(sendAmount);
        if (fee > 0) {
            balances[owner] = balances[owner].add(fee);
            Transfer(msg.sender, owner, fee);
        }
        Transfer(msg.sender, _to, sendAmount);
    }

    /**
     * @dev Gets the balance of the specified address.
     * @param _owner The address to query the the balance of.
     * @return An uint representing the amount owned by the passed address.
     */
    function balanceOf(address _owner) public constant returns (uint balance) {
        return balances[_owner];
    }
}

/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * @dev https://github.com/ethereum/EIPs/issues/20
 * @dev Based oncode by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 */
contract StandardToken is BasicToken, ERC20 {
    mapping(address => mapping(address => uint)) public allowed;

    uint public constant MAX_UINT = 2 ** 256 - 1;

    /**
     * @dev Transfer tokens from one address to another
     * @param _from address The address which you want to send tokens from
     * @param _to address The address which you want to transfer to
     * @param _value uint the amount of tokens to be transferred
     */
    function transferFrom(
        address _from,
        address _to,
        uint _value
    ) public onlyPayloadSize(3 * 32) {
        var _allowance = allowed[_from][msg.sender];

        // Check is not needed because sub(_allowance, _value) will already throw if this condition is not met
        // if (_value > _allowance) throw;

        uint fee = (_value.mul(basisPointsRate)).div(10000);
        if (fee > maximumFee) {
            fee = maximumFee;
        }
        if (_allowance < MAX_UINT) {
            allowed[_from][msg.sender] = _allowance.sub(_value);
        }
        uint sendAmount = _value.sub(fee);
        balances[_from] = balances[_from].sub(_value);
        balances[_to] = balances[_to].add(sendAmount);
        if (fee > 0) {
            balances[owner] = balances[owner].add(fee);
            Transfer(_from, owner, fee);
        }
        Transfer(_from, _to, sendAmount);
    }

    /**
     * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
     * @param _spender The address which will spend the funds.
     * @param _value The amount of tokens to be spent.
     */
    function approve(
        address _spender,
        uint _value
    ) public onlyPayloadSize(2 * 32) {
        // To change the approve amount you first have to reduce the addresses`
        //  allowance to zero by calling `approve(_spender, 0)` if it is not
        //  already 0 to mitigate the race condition described here:
        //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
        require(!((_value != 0) && (allowed[msg.sender][_spender] != 0)));

        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
    }

    /**
     * @dev Function to check the amount of tokens than an owner allowed to a spender.
     * @param _owner address The address which owns the funds.
     * @param _spender address The address which will spend the funds.
     * @return A uint specifying the amount of tokens still available for the spender.
     */
    function allowance(
        address _owner,
        address _spender
    ) public constant returns (uint remaining) {
        return allowed[_owner][_spender];
    }
}

/**
 * @title Pausable
 * @dev Base contract which allows children to implement an emergency stop mechanism.
 */
contract Pausable is Ownable {
    event Pause();
    event Unpause();

    bool public paused = false;

    /**
     * @dev Modifier to make a function callable only when the contract is not paused.
     */
    modifier whenNotPaused() {
        require(!paused);
        _;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is paused.
     */
    modifier whenPaused() {
        require(paused);
        _;
    }

    /**
     * @dev called by the owner to pause, triggers stopped state
     */
    function pause() public onlyOwner whenNotPaused {
        paused = true;
        Pause();
    }

    /**
     * @dev called by the owner to unpause, returns to normal state
     */
    function unpause() public onlyOwner whenPaused {
        paused = false;
        Unpause();
    }
}

contract BlackList is Ownable, BasicToken {
    /////// Getters to allow the same blacklist to be used also by other contracts (including upgraded Tether) ///////
    function getBlackListStatus(
        address _maker
    ) external constant returns (bool) {
        return isBlackListed[_maker];
    }

    function getOwner() external constant returns (address) {
        return owner;
    }

    mapping(address => bool) public isBlackListed;

    function addBlackList(address _evilUser) public onlyOwner {
        isBlackListed[_evilUser] = true;
        AddedBlackList(_evilUser);
    }

    function removeBlackList(address _clearedUser) public onlyOwner {
        isBlackListed[_clearedUser] = false;
        RemovedBlackList(_clearedUser);
    }

    function destroyBlackFunds(address _blackListedUser) public onlyOwner {
        require(isBlackListed[_blackListedUser]);
        uint dirtyFunds = balanceOf(_blackListedUser);
        balances[_blackListedUser] = 0;
        _totalSupply -= dirtyFunds;
        DestroyedBlackFunds(_blackListedUser, dirtyFunds);
    }

    event DestroyedBlackFunds(address _blackListedUser, uint _balance);

    event AddedBlackList(address _user);

    event RemovedBlackList(address _user);
}

contract UpgradedStandardToken is StandardToken {
    // those methods are called by the legacy contract
    // and they must ensure msg.sender to be the contract address
    function transferByLegacy(address from, address to, uint value) public;

    function transferFromByLegacy(
        address sender,
        address from,
        address spender,
        uint value
    ) public;

    function approveByLegacy(address from, address spender, uint value) public;
}

contract TetherToken is Pausable, StandardToken, BlackList {
    string public name;
    string public symbol;
    uint public decimals;
    address public upgradedAddress;
    bool public deprecated;

    //  The contract can be initialized with a number of tokens
    //  All the tokens are deposited to the owner address
    //
    // @param _balance Initial supply of the contract
    // @param _name Token Name
    // @param _symbol Token symbol
    // @param _decimals Token decimals
    function TetherToken(
        uint _initialSupply,
        string _name,
        string _symbol,
        uint _decimals
    ) public {
        _totalSupply = _initialSupply;
        name = _name;
        symbol = _symbol;
        decimals = _decimals;
        balances[owner] = _initialSupply;
        deprecated = false;
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function transfer(address _to, uint _value) public whenNotPaused {
        require(!isBlackListed[msg.sender]);
        if (deprecated) {
            return
                UpgradedStandardToken(upgradedAddress).transferByLegacy(
                    msg.sender,
                    _to,
                    _value
                );
        } else {
            return super.transfer(_to, _value);
        }
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function transferFrom(
        address _from,
        address _to,
        uint _value
    ) public whenNotPaused {
        require(!isBlackListed[_from]);
        if (deprecated) {
            return
                UpgradedStandardToken(upgradedAddress).transferFromByLegacy(
                    msg.sender,
                    _from,
                    _to,
                    _value
                );
        } else {
            return super.transferFrom(_from, _to, _value);
        }
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function balanceOf(address who) public constant returns (uint) {
        if (deprecated) {
            return UpgradedStandardToken(upgradedAddress).balanceOf(who);
        } else {
            return super.balanceOf(who);
        }
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function approve(
        address _spender,
        uint _value
    ) public onlyPayloadSize(2 * 32) {
        if (deprecated) {
            return
                UpgradedStandardToken(upgradedAddress).approveByLegacy(
                    msg.sender,
                    _spender,
                    _value
                );
        } else {
            return super.approve(_spender, _value);
        }
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function allowance(
        address _owner,
        address _spender
    ) public constant returns (uint remaining) {
        if (deprecated) {
            return StandardToken(upgradedAddress).allowance(_owner, _spender);
        } else {
            return super.allowance(_owner, _spender);
        }
    }

    // deprecate current contract in favour of a new one
    function deprecate(address _upgradedAddress) public onlyOwner {
        deprecated = true;
        upgradedAddress = _upgradedAddress;
        Deprecate(_upgradedAddress);
    }

    // deprecate current contract if favour of a new one
    function totalSupply() public constant returns (uint) {
        if (deprecated) {
            return StandardToken(upgradedAddress).totalSupply();
        } else {
            return _totalSupply;
        }
    }

    // Issue a new amount of tokens
    // these tokens are deposited into the owner address
    //
    // @param _amount Number of tokens to be issued
    function issue(uint amount) public onlyOwner {
        require(_totalSupply + amount > _totalSupply);
        require(balances[owner] + amount > balances[owner]);

        balances[owner] += amount;
        _totalSupply += amount;
        Issue(amount);
    }

    // Redeem tokens.
    // These tokens are withdrawn from the owner address
    // if the balance must be enough to cover the redeem
    // or the call will fail.
    // @param _amount Number of tokens to be issued
    function redeem(uint amount) public onlyOwner {
        require(_totalSupply >= amount);
        require(balances[owner] >= amount);

        _totalSupply -= amount;
        balances[owner] -= amount;
        Redeem(amount);
    }

    function setParams(uint newBasisPoints, uint newMaxFee) public onlyOwner {
        // Ensure transparency by hardcoding limit beyond which fees can never be added
        require(newBasisPoints < 20);
        require(newMaxFee < 50);

        basisPointsRate = newBasisPoints;
        maximumFee = newMaxFee.mul(10 ** decimals);

        Params(basisPointsRate, maximumFee);
    }

    // Called when new token are issued
    event Issue(uint amount);

    // Called when tokens are redeemed
    event Redeem(uint amount);

    // Called when contract is deprecated
    event Deprecate(address newAddress);

    // Called if contract ever adds fees
    event Params(uint feeBasisPoints, uint maxFee);
}
```
</details>

コードを部分的に見てみましょう。

```javascript
/**
 *Submitted for verification at Etherscan.io on 2017-11-28
 */

pragma solidity ^0.4.17;
```

Solidityのバージョンは`0.4.17`、`verification`した日付は`2017-11-28`です。
Solidity、`Version 0.4.17`のリリース日が `2017-09-22`なので当時は最新に近いバージョンを使用しているようです。

```javascript
/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) {
            return 0;
        }
        uint256 c = a * b;
        assert(c / a == b);
        return c;
    }

    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        // assert(b > 0); // Solidity automatically throws when dividing by 0
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold
        return c;
    }

    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        assert(b <= a);
        return a - b;
    }

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        assert(c >= a);
        return c;
    }
}
```

Solidityの`uint256`計算において四則演算が正しく行われるようにラッパーする為のライブラリです。


```javascript
/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {
    address public owner;

    /**
     * @dev The Ownable constructor sets the original `owner` of the contract to the sender
     * account.
     */
    function Ownable() public {
        owner = msg.sender;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(msg.sender == owner);
        _;
    }

    /**
     * @dev Allows the current owner to transfer control of the contract to a newOwner.
     * @param newOwner The address to transfer ownership to.
     */
    function transferOwnership(address newOwner) public onlyOwner {
        if (newOwner != address(0)) {
            owner = newOwner;
        }
    }
}
```

`function`に認証を付与するための修飾子`onlyOwner`のコードです。
`transferOwnership`によって `owner` を変更することがでます。

```javascript
/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20Basic {
    uint public _totalSupply;

    function totalSupply() public constant returns (uint);

    function balanceOf(address who) public constant returns (uint);

    function transfer(address to, uint value) public;

    event Transfer(address indexed from, address indexed to, uint value);
}

/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20 is ERC20Basic {
    function allowance(
        address owner,
        address spender
    ) public constant returns (uint);

    function transferFrom(address from, address to, uint value) public;

    function approve(address spender, uint value) public;

    event Approval(address indexed owner, address indexed spender, uint value);
}
```

ERC20のインターフェースです。
現在の仕様「[ERC-20: Token Standard](https://eips.ethereum.org/EIPS/eip-20)」とは一部足りない部分や異なる部分があります。（最終的には全ての要件を満たしています。）

```javascript
/**
 * @title Basic token
 * @dev Basic version of StandardToken, with no allowances.
 */
contract BasicToken is Ownable, ERC20Basic {
    using SafeMath for uint;

    mapping(address => uint) public balances;

    // additional variables for use if transaction fees ever became necessary
    uint public basisPointsRate = 0;
    uint public maximumFee = 0;

    /**
     * @dev Fix for the ERC20 short address attack.
     */
    modifier onlyPayloadSize(uint size) {
        require(!(msg.data.length < size + 4));
        _;
    }

    /**
     * @dev transfer token for a specified address
     * @param _to The address to transfer to.
     * @param _value The amount to be transferred.
     */
    function transfer(address _to, uint _value) public onlyPayloadSize(2 * 32) {
        uint fee = (_value.mul(basisPointsRate)).div(10000);
        if (fee > maximumFee) {
            fee = maximumFee;
        }
        uint sendAmount = _value.sub(fee);
        balances[msg.sender] = balances[msg.sender].sub(_value);
        balances[_to] = balances[_to].add(sendAmount);
        if (fee > 0) {
            balances[owner] = balances[owner].add(fee);
            Transfer(msg.sender, owner, fee);
        }
        Transfer(msg.sender, _to, sendAmount);
    }

    /**
     * @dev Gets the balance of the specified address.
     * @param _owner The address to query the the balance of.
     * @return An uint representing the amount owned by the passed address.
     */
    function balanceOf(address _owner) public constant returns (uint balance) {
        return balances[_owner];
    }
}
```

基本トークン`BasicToken`の実装で、`transfer`と`balanceOf`が実装されています。
`basisPointsRate`と`maximumFee`か設定されている場合、`transfer`時に手数料を`owner`に支払われる仕組みです。

```javascript
/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * @dev https://github.com/ethereum/EIPs/issues/20
 * @dev Based oncode by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 */
contract StandardToken is BasicToken, ERC20 {
    mapping(address => mapping(address => uint)) public allowed;

    uint public constant MAX_UINT = 2 ** 256 - 1;

    /**
     * @dev Transfer tokens from one address to another
     * @param _from address The address which you want to send tokens from
     * @param _to address The address which you want to transfer to
     * @param _value uint the amount of tokens to be transferred
     */
    function transferFrom(
        address _from,
        address _to,
        uint _value
    ) public onlyPayloadSize(3 * 32) {
        var _allowance = allowed[_from][msg.sender];

        // Check is not needed because sub(_allowance, _value) will already throw if this condition is not met
        // if (_value > _allowance) throw;

        uint fee = (_value.mul(basisPointsRate)).div(10000);
        if (fee > maximumFee) {
            fee = maximumFee;
        }
        if (_allowance < MAX_UINT) {
            allowed[_from][msg.sender] = _allowance.sub(_value);
        }
        uint sendAmount = _value.sub(fee);
        balances[_from] = balances[_from].sub(_value);
        balances[_to] = balances[_to].add(sendAmount);
        if (fee > 0) {
            balances[owner] = balances[owner].add(fee);
            Transfer(_from, owner, fee);
        }
        Transfer(_from, _to, sendAmount);
    }

    /**
     * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
     * @param _spender The address which will spend the funds.
     * @param _value The amount of tokens to be spent.
     */
    function approve(
        address _spender,
        uint _value
    ) public onlyPayloadSize(2 * 32) {
        // To change the approve amount you first have to reduce the addresses`
        //  allowance to zero by calling `approve(_spender, 0)` if it is not
        //  already 0 to mitigate the race condition described here:
        //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
        require(!((_value != 0) && (allowed[msg.sender][_spender] != 0)));

        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
    }

    /**
     * @dev Function to check the amount of tokens than an owner allowed to a spender.
     * @param _owner address The address which owns the funds.
     * @param _spender address The address which will spend the funds.
     * @return A uint specifying the amount of tokens still available for the spender.
     */
    function allowance(
        address _owner,
        address _spender
    ) public constant returns (uint remaining) {
        return allowed[_owner][_spender];
    }
}
```

標準トークン`StandardToken`の実装です。
`transferFrom`、`approve`、`allowance`が実装されています。
`transferFrom`は`transfer`と同様に、`basisPointsRate`と`maximumFee`か設定されている場合、手数料を`owner`に支払われる仕組みです。

```javascript
/**
 * @title Pausable
 * @dev Base contract which allows children to implement an emergency stop mechanism.
 */
contract Pausable is Ownable {
    event Pause();
    event Unpause();

    bool public paused = false;

    /**
     * @dev Modifier to make a function callable only when the contract is not paused.
     */
    modifier whenNotPaused() {
        require(!paused);
        _;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is paused.
     */
    modifier whenPaused() {
        require(paused);
        _;
    }

    /**
     * @dev called by the owner to pause, triggers stopped state
     */
    function pause() public onlyOwner whenNotPaused {
        paused = true;
        Pause();
    }

    /**
     * @dev called by the owner to unpause, returns to normal state
     */
    function unpause() public onlyOwner whenPaused {
        paused = false;
        Unpause();
    }
}
```

一時停止`Pausable`を実装しています。
`whenNotPaused`、`whenPaused`修飾子で操作可能です。

```javascript
contract BlackList is Ownable, BasicToken {
    /////// Getters to allow the same blacklist to be used also by other contracts (including upgraded Tether) ///////
    function getBlackListStatus(
        address _maker
    ) external constant returns (bool) {
        return isBlackListed[_maker];
    }

    function getOwner() external constant returns (address) {
        return owner;
    }

    mapping(address => bool) public isBlackListed;

    function addBlackList(address _evilUser) public onlyOwner {
        isBlackListed[_evilUser] = true;
        AddedBlackList(_evilUser);
    }

    function removeBlackList(address _clearedUser) public onlyOwner {
        isBlackListed[_clearedUser] = false;
        RemovedBlackList(_clearedUser);
    }

    function destroyBlackFunds(address _blackListedUser) public onlyOwner {
        require(isBlackListed[_blackListedUser]);
        uint dirtyFunds = balanceOf(_blackListedUser);
        balances[_blackListedUser] = 0;
        _totalSupply -= dirtyFunds;
        DestroyedBlackFunds(_blackListedUser, dirtyFunds);
    }

    event DestroyedBlackFunds(address _blackListedUser, uint _balance);

    event AddedBlackList(address _user);

    event RemovedBlackList(address _user);
}
```

ブラックリスト`BlackList`の実装です。
`addBlackList`、`removeBlackList`、`destroyBlackFunds`が実装されています。
ブラックリストの追加、削除、資産の破棄が`owner`によって実行可能です。

```javascript
contract UpgradedStandardToken is StandardToken {
    // those methods are called by the legacy contract
    // and they must ensure msg.sender to be the contract address
    function transferByLegacy(address from, address to, uint value) public;

    function transferFromByLegacy(
        address sender,
        address from,
        address spender,
        uint value
    ) public;

    function approveByLegacy(address from, address spender, uint value) public;
}
```

アップグレード`UpgradedStandardToken`のインターフェース実装です。

```javascript
contract TetherToken is Pausable, StandardToken, BlackList {
    string public name;
    string public symbol;
    uint public decimals;
    address public upgradedAddress;
    bool public deprecated;

    //  The contract can be initialized with a number of tokens
    //  All the tokens are deposited to the owner address
    //
    // @param _balance Initial supply of the contract
    // @param _name Token Name
    // @param _symbol Token symbol
    // @param _decimals Token decimals
    function TetherToken(
        uint _initialSupply,
        string _name,
        string _symbol,
        uint _decimals
    ) public {
        _totalSupply = _initialSupply;
        name = _name;
        symbol = _symbol;
        decimals = _decimals;
        balances[owner] = _initialSupply;
        deprecated = false;
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function transfer(address _to, uint _value) public whenNotPaused {
        require(!isBlackListed[msg.sender]);
        if (deprecated) {
            return
                UpgradedStandardToken(upgradedAddress).transferByLegacy(
                    msg.sender,
                    _to,
                    _value
                );
        } else {
            return super.transfer(_to, _value);
        }
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function transferFrom(
        address _from,
        address _to,
        uint _value
    ) public whenNotPaused {
        require(!isBlackListed[_from]);
        if (deprecated) {
            return
                UpgradedStandardToken(upgradedAddress).transferFromByLegacy(
                    msg.sender,
                    _from,
                    _to,
                    _value
                );
        } else {
            return super.transferFrom(_from, _to, _value);
        }
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function balanceOf(address who) public constant returns (uint) {
        if (deprecated) {
            return UpgradedStandardToken(upgradedAddress).balanceOf(who);
        } else {
            return super.balanceOf(who);
        }
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function approve(
        address _spender,
        uint _value
    ) public onlyPayloadSize(2 * 32) {
        if (deprecated) {
            return
                UpgradedStandardToken(upgradedAddress).approveByLegacy(
                    msg.sender,
                    _spender,
                    _value
                );
        } else {
            return super.approve(_spender, _value);
        }
    }

    // Forward ERC20 methods to upgraded contract if this one is deprecated
    function allowance(
        address _owner,
        address _spender
    ) public constant returns (uint remaining) {
        if (deprecated) {
            return StandardToken(upgradedAddress).allowance(_owner, _spender);
        } else {
            return super.allowance(_owner, _spender);
        }
    }

    // deprecate current contract in favour of a new one
    function deprecate(address _upgradedAddress) public onlyOwner {
        deprecated = true;
        upgradedAddress = _upgradedAddress;
        Deprecate(_upgradedAddress);
    }

    // deprecate current contract if favour of a new one
    function totalSupply() public constant returns (uint) {
        if (deprecated) {
            return StandardToken(upgradedAddress).totalSupply();
        } else {
            return _totalSupply;
        }
    }

    // Issue a new amount of tokens
    // these tokens are deposited into the owner address
    //
    // @param _amount Number of tokens to be issued
    function issue(uint amount) public onlyOwner {
        require(_totalSupply + amount > _totalSupply);
        require(balances[owner] + amount > balances[owner]);

        balances[owner] += amount;
        _totalSupply += amount;
        Issue(amount);
    }

    // Redeem tokens.
    // These tokens are withdrawn from the owner address
    // if the balance must be enough to cover the redeem
    // or the call will fail.
    // @param _amount Number of tokens to be issued
    function redeem(uint amount) public onlyOwner {
        require(_totalSupply >= amount);
        require(balances[owner] >= amount);

        _totalSupply -= amount;
        balances[owner] -= amount;
        Redeem(amount);
    }

    function setParams(uint newBasisPoints, uint newMaxFee) public onlyOwner {
        // Ensure transparency by hardcoding limit beyond which fees can never be added
        require(newBasisPoints < 20);
        require(newMaxFee < 50);

        basisPointsRate = newBasisPoints;
        maximumFee = newMaxFee.mul(10 ** decimals);

        Params(basisPointsRate, maximumFee);
    }

    // Called when new token are issued
    event Issue(uint amount);

    // Called when tokens are redeemed
    event Redeem(uint amount);

    // Called when contract is deprecated
    event Deprecate(address newAddress);

    // Called if contract ever adds fees
    event Params(uint feeBasisPoints, uint maxFee);
}
```

メイントークン`TetherToken`の実装です。
一時停止`Pausable`、標準トークン`StandardToken`、ブラックリスト`BlackList`を継承しています。
ERC20以外の機能は、アップグレード`deprecate`、発行`issue`、償還`redeem`、手数料設定`setParams`が実装されており、`owner`によって実行可能です。


# まとめ（Conclusion）

ERC20の基本的な機能はもちろん、いろいろな工夫がされています。
ERC20以外の機能としては、以下があげられます。

- 認証（`onlyOwner`）
- 一時停止（`paused`）
- 手数料（`fee`）
- アップグレード（`upgrade`）
- ブラックリスト（`blacklist`）
- 発行（`issue`）
- 償還（`redeem`）

これらの機能は最近のトークンでも実装されている機能です。
初期のコードなのでシンプルなコードとは言えませんが、かなり考えられて作られたことがわかります。