AltGr key not working anywhere in X

[Ville V Sinkko <vsinkko@cc.helsinki.fi>]

I just set up Debian for the first time to replace the erratically-behaving
MDK8.1 I had. Most other thing are fine, but I can't get AltGr to work in
X programs. Since I use a Finnish keymap, I can't produce stuff like square
brackets, which makes it practically impossible to, say, code C.

What can I do to fix it? The only instructions for fixing this that I've
managed to find simply told me to make sure that Option "RightAlt" "ModeShift"
is present in XF86Config. It is, to no visible effect.





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