kit vs. app

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Please explain or point me to the documentation that explains the 
difference between kit() and app().
I have not been able to find it.

In most of the examples, main contains:
Gtk::Main kit(argc, argv);
. . .
kit.run();

However, in Chapter 21. Timeouts, I/O and Idle Functions:
Gtk::Main app(argc, argv[]);
. . .
Gtk::Main::run(example);

Thanks!