1.利用库保证合约安全，出错的时候检查库就可以

library Balances {
    function send(mapping(address => uint256) storage balances, address from, address to, uint amount) internal {
        require(balances[from] >= amount);
        require(balances[to] + amount >= balances[to]);
        balances[from] -= amount;
        balances[to] += amount;
    }
}
contract Token {
    mapping(address => uint256) balances;
    using Balances for *;
    mapping(address => mapping (address => uint256)) allowed;
    event Transfer(address from, address to, uint amount);
    event Approval(address owner, address spender, uint amount);
    function balanceOf(address tokenOwner) public constant returns (uint balance) {
        return balances[tokenOwner];
    }
    function transfer(address to, uint amount) public returns (bool success) {
        balances.send(msg.sender, to, amount);
        emit Transfer(msg.sender, to, amount);
        return true;
    }
    function transferFrom(address from, address to, uint amount) public returns (bool success) {
        require(allowed[from][msg.sender] >= amount);
        allowed[from][msg.sender] -= amount;
        balances.send(from, to, amount);
        emit Transfer(from, to, amount);
        return true;
    }
    function approve(address spender, uint tokens) public returns (bool success) {
        require(allowed[msg.sender][spender] == 0, "");
        allowed[msg.sender][spender] = tokens;
        emit Approval(msg.sender, spender, tokens);
        return true;
    }
}