contract ERC20 is ERC20Basic {
    /* 获取允许spender还能提取token的额度 */
    function allowance(address owner, address spender) public view returns (uint256);
    /* 批准spender账户从自己的账户转移value个token，可分多次转移 */
    function transferFrom(address from, address to, uint256 value) public returns (bool);
    /* 与approve搭配使用，approve批准之后，调用transferFrom来转移token */
    function approve(address spender, uint256 value) public returns (bool);
    /* 当调用approve成功时，一定要触发Approval事件 */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}
/* ERC20标准 */
function approve(address spender, uint256 value) public returns (bool);

/* 与approve搭配使用，approve批准之后，调用transferFrom来转移token */