(defun list_even_items (L)
  (defun even (L n)
    (if (null L)
      (if (= 0 (mod n 2))
        T
        NIL)
      (even (cdr L) (1+ n))))
    (even L 0))

(defun sum_atoms (L)
  (defun sum (L n)
    (if (null L)
      n
      (if (numberp (car L))
        (sum (cdr L) (+ n (car L)))
        (sum (cdr L) n))))
    (sum L 0))

(defun max_depth (L &optional (md 0))
  (defun depth (L n)
    (if (atom L)
      (setq md (max n md))
      (loop for elem in L
        do (depth elem (1+ n)))))
  (depth L md)
  md)

(defun max_elem (L)
  (defun elems (L)
    (if (atom L)
      (setq me (max me L))
      (loop for elem in L
        do (elems elem))))
  (setq me (car L))
  (elems L)
  me)

(defun eval_expr (L)
  (defun calc (L)
    (cond ((not (listp L)) L)
      ((null (cdr L)) (calc (car L)))
      ((eq (cadr L) '*) (calc (cons (* (calc (car L)) (calc (caddr L))) (cdddr L))))
      ((eq (cadr L) '/) (calc (cons (/ (calc (car L)) (calc (caddr L))) (cdddr L))))
      ((eq (cadr L) '+) (+ (calc (car L)) (calc (cddr L))))
      (T (- (calc (car L)) (calc (cddr L)))))
  (cond ((not(listp L)) "not list")
    ((null L) "the list is empty")
    (T (calc L)))))