backquote trivia, for you history fans.

[George J. Carrette <GJC at MIT-MC>]

The PDP-10 Maclisp implementation of Backquote works at macroexpansion
time as you wanted. This was done for ease of GRINDEF'ing.
The Lispmachine implementation provides special synonyms for LIST, CONS,
LIST*, APPEND, etc, and uses a simple pattern matcher to create a cannonical
pretty form for GRINDEF. The NIL implementation uses code lifted from
code from which the Multics Maclisp backquote was derived, which is also
related in some way to the Lispmachine backquote.

In all implementations there are "car-position" markers for "," ",@" and ",."
and some simple entry to the "BACKQUOTIFY" function called by "`"
either at read or eval times.

Q: What is the deepest nesting of backquote found to arise in practice?

-gjc