contract Token {
    /// token总量，默认会为public变量生成一个getter函数接口，名称为totalSupply().
    uint256 public totalSupply;
 
    /// 获取账户_owner拥有token的数量
    function balanceOf(address _owner) constant returns (uint256 balance);
 
    //从消息发送者账户中往_to账户转数量为_value的token
    function transfer(address _to, uint256 _value) returns (bool success);
 
    //从账户_from中往账户_to转数量为_value的token，与approve方法配合使用
    function transferFrom(address _from, address _to, uint256 _value) returns  (bool success);
 
    //消息发送账户设置账户_spender能从发送账户中转出数量为_value的token
    function approve(address _spender, uint256 _value) returns (bool success);
 
    //获取账户_spender可以从账户_owner中转出token的数量
    function allowance(address _owner, address _spender) constant returns  (uint256 remaining);
 
    //发生转账时必须要触发的事件 
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
 
    //当函数approve(address _spender, uint256 _value)成功执行时必须触发的事件
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}
function allowance(address _owner, address _spender) constant returns  (uint256 remaining);

//获取账户_spender可以从账户_owner中转出token的数量