此实现是封闭的：在您编译FulfillmentGenerator类以后，在不编辑源代码和重新编译的情况下，您无法将invoice方法扩展到新的类型。

And this implementation is closed: After you compile the FulfillmentGenerator class, you can't extend the invoice method to new types without editing the source code and recompiling.

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静态成员中的封闭类型参数。

Enclosing type parameters in static members.

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采用平面四边形为基本封闭形建立位移方程，给出了所有四个基本类型的解。

A planar quadrilateral is adopted as the basic loop in building up the displacement equations, and four basic types of solutions are given.

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