" The function flatten takes in as input a list and returns   "
" a flat list consisting of all the atoms present in the list "

(defun flatten (lst)
       (if (null lst) 
	   lst
	   (if (atom (car lst)) 
	       (append (list (car lst)) (flatten (cdr lst)))
	       (append (flatten (car lst)) (flatten (cdr lst)))
)      )   )

(defun factorial (n)
       (if (= n 0) 
           1
	   (* n (factorial (- n 1)))
)      )


(defun count_atoms (lst)
       (if (null lst) 
	   0
	   (if (atom (car lst)) 
	       (+ 1 (count_atoms (cdr lst)))
	       (+ (count_atoms (car lst)) (count_atoms (cdr lst)))
)      )   )

(defun apply_subst (x y lst)
       (if (null lst)
           '()
	   (if (equal y (car lst))
               (cons x (apply_subst x y (cdr lst)))
               (if (atom (car lst))
                   (cons (car lst) (apply_subst x y (cdr lst)))
                   (cons (apply_subst x y (car lst)) 
                         (apply_subst x y (cdr lst)))
)      )   )   )