Scala可以区分主构造函数和0个或多个辅助构造函数。

Scala distinguishes between a primary constructor and zero or more auxiliary constructors.

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它调用主构造函数，并且传入Nil (Nil表示了一个空的List对象)。

It calls the primary constructor with Nil (which represents an empty List object).

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主构造函数，类的主题，有一个参数列表来接受标签字符串和回调函数的列表。

The primary constructor, which is the body of the entire class, has a parameter list that takes a label string and a list of callback functions.

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