GTK key theme

[Shaun McCance <shaunm wolfram com>]

I'm having trouble constructing a GTK key theme.  I'm trying to set up a
theme to have basic Emacs cursor-movement bindings, but bound to mod2
instead of control.  I copied the Emacs theme and stripped it down to
just one keybinding.  So I have

binding "gtk-super-text-entry"
{
  bind "<mod2>b" { "move-cursor" (logical-positions", -1, 0) }
}

class "GtkEntry" binding "gtk-super-text-entry"
class "GtkTextView" binding "gtk-super-text-entry"

But when I open a GTK app and hit "<mod2>b", I just get a "b".  If I
change "<mod2>b" to "<ctrl>b" (as it is in the Emacs key theme), then it
works.  I have mod2 bound to my Windows keys as Super_L and Super_R. 
Any ideas on why this isn't working?

Thanks,
Shaun