split string function in common lisp

split-str.l

(defun split-str (string &optional (separator " "))
  (split-str-1 string separator))

(defun split-str-1 (string &optional (separator " ") (r nil))
  (let ((n (position separator string
		     :from-end t
		     :test #'(lambda (x y)
			       (find y x :test #'string=)))))
    (if n
	(split-str-1 (subseq string 0 n) separator (cons (subseq string (1+ n)) r))
      (cons string r))))

This only seems to work with single char separators.