[plt-scheme] lambda argument lists that contain only a rest argument

Why is this OK:
(define (rest-only . r)
  r)

But this can't even be read into the interpreter:
(define rest-only
  (λ ( . r)
    r))

...and what can I do about it?  I want to be able to make a lambda
that may take 0 or more arguments.

Thanks!
-Ethan Herdrick
herdrick.blogspot.com