systool 源代码，他可以解析表达式的值，还可以自定义函数（最多可以带3个参数），自定义变量。很好用，不需要安装，直接引用-systool source code, he can resolve the value of the expression, you can also custom functions (with up to 3 parameters), the custom variable. Useful, no need to install a direct quote