Any language looks a lot harder when one doesn't use the common indentation conventions of a language. When one follows them of Lisp, one sees how it expresses a syntax-tree structure quite readily (note, this isn't quite right because the preview lies a little; the r's should align with the fns in the recursive quicksort argument):
(defun quicksort (lis)
(if (null lis)
nil
(let* ((x (car lis))
(r (cdr lis))
(fn (lambda (a)
(< a x))))
(append (quicksort (remove-if-not fn
r))
(list x)
(quicksort (remove-if fn
r))))))
