ねえ、これってクラス作成時は必ずしも継承のためのスーパークラスを指定する必要は無いって事？
NSObjectみたいなの無いの？？

Subclasses include their superclass name after their class name, separated by a colon.
There is no requirement for classes to subclass any standard root class, so you can include or omit a superclass as needed.