IF macro

[MOON at MIT-MC (David A. Moon)]

I don't use this myself, but I thought that if we can have 100
pieces of mail about lsubr cons and backquote this may be worthy
of discussion.

There are extant at least a half-dozen different incompatible IF
macros.  Since so many people seem to like it, it would seem to
be a good idea to standardize on one that would be compatible across
all programs.  

The one I like the best is Bernie Greenberg's.  It looks like:
	(IF <predicate>
              <then1> <then2> ....
            ELSE
              <else1> <else2> ....)
The keyword ELSE and the else-forms may be omitted.  The implied
PROGN seems so useful as to counteract the slight cruftiness of
having an extra embedded keyword.

For myself, I would probably continue to use COND.