post you are run length encoding in lisp

critique mine pl0x:

(defun rle (str) ;run length encoding
(let ((i 0) c count out)
(while (< i (length str))
(if (eq c (elt str i))
(setq count (1+ count))
(when c
(setq out (append out (list (cons c count)))))
(setq c (elt str i))
(setq count 1))
(setq i (1+ i)))
(when c (setq out (append out (list (cons c count)))))
out))

>>6
how would you deal with the inefficiency of checking if there is no previous (char . count) to output? doing the test always seems slow.