input$ and text collide?

Function check(check$)
If check$="quit"
quit = 1
EndIf 
End Function

************
Then after each check() call put:

if quit=1 then goto quitlabel

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Not elegant, by any means, but it works :)
The problem with "undefined label" is that .quitlabel is a local label and you were trying to reference it from within a function. Being local, the function couldn't see it and complained.