(defun permutations (bag)
  "Return a list of all the permutations of the input."
  ;; If the input is nil, there is only one permutation:
  ;; nil itself
  (if (null bag) 
      '(())
      ;; Otherwise, take an element, e, out of the bag.
      ;; Generate all permutations of the remaining elements,
      ;; And add e to the front of each of these.
      ;; Do this for all possible e to generate all permutations.
      (mapcan #'(lambda (e)
                  (mapcar #'(lambda (p) (cons e p))
                          (permutations
                            (remove e bag :count 1))))
              bag)))