当在会话中第一次引用时，全局变量将进行实例化来获得其默认值。
A global variable is instantiated to its default value when it is first referenced in the session.
可以将泛型类型看作是类型“函数”；它们通过类型变量进行参数化，这些类型变量可以根据上下文用各种类型参数进行实例化。
Generic types can be thought of as type "functions"; they are parameterized by type variables that can then be instantiated with various type arguments depending on context.
以下描述的斜体词就是源代码实例化变量的名字(也见于代码清单5、6、7和8)。
The words in italics in the following descriptions are the names of instantiated variables in the source code (also see Listings 5, 6, 7, and 8, which follow).
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