(defun square(x) (* x x))

(defun myabs(x)
   (if (< x 0) (- x) x)
)

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

(defun rev(l)
   (if (null l)
       nil
       (append (rev (cdr l)) (list (car l)))
   )
)

(defun flatten(x)
   (cond ( (null x) nil )
         ( (atom (car x)) (cons (car x) (flatten (cdr x))) )
         ( T (append (flatten (car x)) (flatten (cdr x))) )
   )
)