从 6.1 开始 pi * 0..m - 范围实例（scope instance）的管理设置

Since: 6.1 pi * 0..m - Administrative settings for scope instances

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如果没有对参数设定显式的范围，那么缺省情况下范围是Object，这意味着我们不能调用范围实例在Object中未曾出现的任何方法。

If we place no explicit bound on the parameter, then by default the bound is Object, meaning that we can't call any methods on an instance of the bound that don't appear in class Object.

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如果您适当地限制了变量的范围并且减少实例变量的使用，则可以减少此类对象保留。

If you scope variables appropriately and reduce the use of instance variables, you can reduce this kind of object retention.

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