Default value: (not (eq window-system 'x))
Whether we allow reversing.
This is useful if you cannot use “shift-next” for “previous” (as wanted for pedal use). These seem to work OK on X but not on an old Windows Emacs; not sure about other platforms/terminals yet.
Although the platform that needed this has now moved on, the feature is kept for possible use with very limited input devices such as mouthswitches, thus helping to make GNUemacs accessible even to quadriplegics.