当在表达式中使用. .时，它能指示表达式从层次结构中的当前节点上提一层(这亦类似于unix操作系统内目录的处理)。
When you use.. in an expression, it instructs the expression to work up one level from the current node in the hierarchy (again, similar to working with directories in UNIX operating systems).
当在表达式中使用. .时，它能指示表达式从层次结构中的当前节点上提一层(这亦类似于unix操作系统内目录的处理)。
When you use.. in an expression, it instructs the expression to work up one level from the current node in the hierarchy (again, similar to working with directories in UNIX operating systems).
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