() as an expression

[Jason Coughlin <image.soe.clarkson.edu!jk0@uunet.uu.net>]

The Revised^3 Report says that () is an illegal expression.  It must
be quoted.  However, MIT-Scheme and PC-Scheme both allow it, ie:

MIT-SCHEME => ()
()

MIT-SCHEME => 

Why is () an invalid expression?  It seems to me that it is a constant.

(eq? #t #t) => #t
(eq? #f #f) => #t
(eq? '() '()) => #t

now why isn't () considered a constant, when it really IS a constant?
-- 
Jason Coughlin ( jk0@sun.soe.clarkson.edu , jk0@clutx )
"Every jumbled pile of person has a thinking part that wonders what the
part that isn't thinking isn't thinking of." - They Might Be Giants