How can I use AltGr as the Gnome Shell/Mutter mouse modifier?

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http://askubuntu.com – I have the right Alt key bound to , but mouse left-handed and would also like to use it for moving windows (in Gnome Shell/Mutter). I have tried setting /org/gnome/desktop/wm/preferences/mouse-button-modifier to <AltGr>, but it does not work. (HowTos)