Empty is empty.
Singleton1 is not empty.
Singleton2 is not empty.
NonSingleton1 is not empty.
NonSingleton2 is not empty.

Empty is not a singleton.
Singleton1 is a singleton with element 1.
Singleton2 is a singleton with element 2.
NonSingleton1 is not a singleton.
NonSingleton2 is not a singleton.