gistfile1.lisp

(defun next-candidate (candidate)
  (labels ((iter-helper (xs &optional prev-item)
             (if (null  (first xs))
                 (rotatef (first xs) prev-item)
                 (iter-helper (rest xs) (first xs)))))
               (when (null (first candidate))
             (signal 'no-more-combinations))
    (iter-helper candidate)))

(defvar test (list 1 nil 2))
(next-candidate test)
;; => (1 1 2)
;; => (nil 1 2)