(defun treesum (x)
    (cond
        ((equal x nil) 0)
        ((integerp x) x)
        (t (+ (treesum (first x)) (treesum (rest x))))
    )
)

(defun square-elements1 (x)
    (let ((a nil))
        (dolist (y x) (push (* y y) a))
        (reverse a)
    )
)

(defun square-elements2 (x)
    (mapcar #'(lambda (y) (* y y)) x)
)