External-syntax for number radices

[GLS at MIT-AI (Guy L. Steele, Jr.)]

For what it is worth, if I had my free and random choice
in the matter I would still favor the nnn[rr] syntax for
arbitrary-radix numbers, and let [ be a macro character only
when not in the middle of such a number.  However, I am
still willing to settle for the #X/#O compromise
(though both #O and #Q have the bug that the key character
looks like a digit).  Sigh.