This function returns a pointer to the value in the environment or NULL if there is no match.
这个函数会返回一个 指向环境中的值的指针，或者如果不匹配则返回 NULL。
The function finds the first node to match the given XPath and extracts its text value.
该函数查找匹配给定xpath的第一个节点并抽取其文本值。
The callback function is invoked for each match, and the match value is passed as the argument to the callback.
每个匹配都会调用回调函数，并把匹配值作为参数传递给它。
The callback function is invoked for each match, and the match value is passed as the argument to the callback.
每个匹配都会调用回调函数，并把匹配值作为参数传递给它。
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