Simple e-lisp question

[Eric Lilja <mindcooler@gmail.com>]

Hi, I want to write an elisp-function switch2 that should switch the 
first two elements in a list. I came up with this:

(defun switch2 (x)
   (append (list (second x) (first x)) (nthcdr 2 x))
)
(switch2 '(a b c d)) ; Yields (b a c d)
(switch2 '(a b)) ; Yields (b a)
(switch2 '(a)) ; Yields (nil a)
(switch2 '()) ; Yields (nil nil)

The problem is how it handles a list with only one element and an empty 
list. I'm not sure how it should handle only one element, maybe return 
an unmodified list or an empty list? If an empty list is given the 
result should be an empty list.

How can I fix my swith2 to cope better with the last two calls above and 
can I use the more fundamental list functions if you know what I mean 
and avoid nthcdr altogether?

- Eric Lilja

PS. I don't know lisp, heh, just found this old exercise in my papers. DS.