要记住 AST 类在包外是不可见的， 还有就是客户机（最终）只会传入它们想求值的表达式的一个字符串表示。

Remember that the AST classes aren't visible outside of this package at all and that clients will (eventually) simply pass in a String representation of the expression they want evaluated.

youdao

要对表达式求值，该结构中的每个类都有一个 evaluate() 方法，它负责返回表达式的值。

To evaluate the expression, each class in this structure has an evaluate() method, which is responsible for returning its value.

youdao