Rails同时为许多其他对象提供了合理的blank ？定义：true, false，空数组及哈希表，数值类型，甚至是Object类本身。

Rails provides reasonable definitions of blank? For several other objects too: true, false, empty arrays or hashes, numeric types, and even the object class.

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Python 3现支持能代表数值类的ABC的类型层次结构。

Python 3 now supports a type hierarchy of ABCs that represent numeric classes.

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建立一个绑定类型模板并将其命名为ShelvesList通过设定参数值t为Shelf经其与类List联系起来。

Create a template binding class named ShelvesList with a binding to the List class by setting the parameter t to Shelf.

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