不支持探测重载和多态的函数。
Probing of overloaded and polymorphic functions is not supported.
协议允许您在先前存在的类型上定义多态函数。
Protocols allow you to define polymorphic functions over preexisting types.
就这样，您具有多态invoice和manifest函数，可以在一个Order或一个Contract上调用且作出正确操作。
And with that, you have polymorphic invoice and manifest functions that can be called on either an Order or a Contract and that will do the right thing.
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