One other thing... it strikes me that cage uses this restriction on the
letters to help give clarity to the poem as a whole and to highlight the
featured letter since there are no other occurances of it close by. Also
the rule of no other capital letters seems to be for the same clarity
and aesthetic. Would you say that is correct rod? Or do you feel there
are other reasons for these rules?
Then it seems that you could also have the the spine make a curve,
diagnal or other shape if you wanted.
cecil