The evaluate() method takes two arguments: a string XPath, a DOM tree to evaluate that XPath against, and an XPath constant indicating the return type.
evaluate() 方法拥有两个参数：一个字符串 XPath，一个 DOM 树（用来针对其计算该 XPath），还有一个 XPath 常量表示返回类型。
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Evaluate is called with the XPath string passed into the method, the DOM tree you built in the class's constructor, and the constant indicating to return results as a list of nodes.