该方法结构简单，而且不选主元。
There is very simple method which does not require choosing Primary element.
如果目标是一个属性引用：引用中的主元表达式被求值。
If the target is an attribute reference: the primary expression in the reference is evaluated.
如果目标是一个属性引用：引用中的主元表达式被求值。
If the target is an attribute reference: the primary expression in the reference is evaluated.
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