[h-e-w] How can I bind to C-?

[David Vanderschel]

I've tried every key-binding variation with which I am
familiar with no luck.

eg., "\C-?", [(control ??)], [(control shift /)],
[?\C-?], ...

(I was experimenting with that redo.el package (which
does strike me as mostly overkill, since undo's are
themselves undoable after moving the point) and DEL
struck me as a bad key to which to bind redo.  Since I
have undo on C-/, C-S-/ seemed like the obvious
choice.  But I don't seem to be able to make it work,
and now I want to solve the mystery.)

Regards,
  David V.