...，类只能根据命名了的父类（superclass,它不能是任意的类运算式）定义，而且只能使用某些特定类型的类约束（class restriction）。类之间的等价关系（equivalence）和子类关系只能在具名类（named class）上做声明，不能应用于任意的类运算式。
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Class of Restriction 限制等级
Class Of Service Restriction 服务等级限制
storage class attribute restriction [计] 存储类属性限制
trunk restriction class 中继线限制分类
In modeling terms, you would most usually expect to see a UML component here; however, such a restriction seems arbitrary, and so the metaclass is noted as class for more flexibility.
在建模的术语里，你可能最经常期望见到的一个UM L组件。然而，这样的一个约束似乎是武断的，所以为了更大的灵活性，元类被指明作为一个类。
In this case, the restriction selects all resources that have an issue property where the value of that property is of class issue.
在这个例子里，约束条件选择所有具有issue特性并且该特性的值属于类issue的资源。
This restriction is lifted in Groovy; you can change the values of variables in a containing scope from within an anonymous inner class.
Groovy则消除了这种限制，在Groovy中，匿名内部类可以修改外部的变量值。
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