;; demonstrates functional thinking in Common Lisp

(defun combinations ( alist n &optional (list-so-far nil) )
  (cond
    ((= (length list-so-far) n) (print (reverse list-so-far)))
    ((null alist) nil)
    (t
       (combinations (rest alist) n (cons (first alist) list-so-far))
       (combinations (rest alist) n list-so-far)
    )
  )
)

;; demonstrates rest recursion

(defun my-length ( alist )
  (cond
    ((null alist) 0)
    (t
      (+ 1 (my-length (rest alist)))
    )
  )
)

;; demonstrates first rest recursion

(defun count-all ( alist )
  (cond
    ((null alist) 0)
    ((atom alist) 1)
    (t
      (+ (count-all (first alist)) (count-all (rest alist)))
    )
  )
)

;; demonstrates first rest recursion, developed with help of students

(defun flatten (alist)
  (cond
    ((null alist) nil)
    ((atom alist) (list alist))
    (t 
       (append (flatten (first alist)) (flatten (rest alist)))
    )
  )
)