如果这样一个程序以串行方式执行，就可以把它看作一个子过程的调用链，一个接一个的进行求值。
If this program is carried out serially, it could be viewed as a chain of subroutine calls, evaluated one after another.
要完成整个路径位置表达式的求值，所有元素都必须步进，并匹配第一次出现的子元素named - USPrice。
To complete the evaluation of the entire path location expression, each of these elements will be stepped into and the first occurrence of an expected child element named - USPrice matched on.
Rope实现通常也将延迟对大型子串操作的求值，它的作法是引入子串节点。
Rope implementations also often defer evaluation of large substring operations by introducing a substring node.
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