(DEFUN ONLEVEL(L S)
 (COND ((null S) 0)
       ((AND (> L '0) (ATOM (CAR S))) (ONLEVEL L (CDR S)))
       ((> L '0) (+ (ONLEVEL (- L 1) (CAR S)) (ONLEVEL L (CDR S))))
       (1 (COUNT_S S))
 )
)

(DEFUN COUNT_S(S)
 (COND ((null S) 0)
       ((ATOM (CAR S)) (COUNT_S (CDR S)))
       (1 (+ 1 (COUNT_S (CDR S))))
 )
)
 
 
(PRINT (ONLEVEL '2 '(1 (2 (4 (5 6))) 3 5 (6) (7 (8 (10) (11))))))