(defun combos (list)
  (let* ((a (car list))
         (d (cdr list))
         (s (and d (combos d)))
         (v (loop for c in s collect (cons a c))))
    (cons (list a) (append s v))))

;; Requires SRFI 26
(define (combos lst)
  (if (null? lst) '(())
      (let* ((a (car lst))
             (d (cdr lst))
             (s (combos d))
             (v (map (cut cons a <>) s)))
        (append s v))))

(defun combos (list)
  (if (null list) '(nil)
      (let* ((a (car list))
             (d (cdr list))
             (s (combos d))
             (v (mapcar (lambda (x) (cons a x)) s)))
        (append s v))))

(defun subsets (list)
  "Return a list of the subsets of LIST."
  (if (null list) '(nil)
    (let ((e (car list))
          (ss (subsets (cdr list))))
      (append ss (loop for s in ss collect (cons e s))))))

(defun non-empty-subsets (list)
  "Return a list of the non-empty subsets of LIST."
  (cdr (subsets list)))