...，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 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。
By its subclassing from this restriction, the RelevantResource class is a sort of virtual class that includes a set of all resources that meet the restriction.
通过从这一约束条件划分子类， RelevantResource 类成为一种包含一个所有满足该约束条件资源集合的虚拟类。
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