如何通过单表继承访问父属性？
How to access parent attributes via single table inheritance?
每个运算式内容都具有会在堆叠上置放下一个运算式内容的父属性。
Each expression context has a parent property that locates the next expression context on the stack.
然后，父容器使用这些属性度量它本身。
The parent container then uses these properties to measure itself.
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