[plt-scheme] Exiting a Scheme GUI-program

I'm making a program with PLT Scheme and I've created my window using (new
frame%) and added a menu with (new menu-bar%).
The simple question is: how do I make a menu-item to close my program?
Should I call some kind of yield? I can't this information :(

My code so far for the buttun is only:

(define quit-item (new menu-item% [parent file-menu] [label "Quit"]
                            (callback (lambda (button event)
                                        'Q))))

 - Moberg
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