Описать функцию, которая по атому и списку выдает множество всех атомов в списке

(defun dive-elms (n w &aux (a (car w)) (d (cdr w)))
   (cond ((null w) nil)
         ((= n 0) w)
         ((atom a) (dive-elms n d))
         ((nconc (dive-elms (1- n) a) (dive-elms n d)))))
 
(defun next (w z &aux (a (car w)) (d (cdr w)))
  (cond ((null w) nil)
        ((eq a z) (cons (car d) (next (cdr d) z)))
        ((next d z))))
 
(defun dive-next (n w z)
  (next (dive-elms n w) z))
 
> (dive-next 0 '(a 1 (b 10 (c 100)) (b 10 (c 100 (d 1000)) e)) 'a)
(1)
> (dive-next 1 '(a 1 (b 10 (c 100)) (b 10 (c 100 (d 1000)) e)) 'b)
(10 10)
> (dive-next 2 '(a 1 (b 10 (c 100)) (b 10 (c 100 (d 1000)) e)) 'c)
(100 100)
> (dive-next 3 '(a 1 (b 10 (c 100)) (b 10 (c 100 (d 1000)) e)) 'd)
(1000)

(defun task (x lst n &optional (p 0))
  (let ((res nil) (tmp nil))
    (cond ((= n p) 
           (dolist (i lst (reverse (flatten res)))
             (when (and (eq (car tmp) x) (atom i)) (push i res))
             (when (atom i) (push i tmp)))) 
          ((< p n)
           (dolist (i lst (reverse (flatten res)))
             (when (listp i) (push (task x i n (+ p 1)) res))))
          (t nil))))
 
==> task
 
(task 'a '(a 1 (b 10 (c 100)) (b 10 (c 100 (d 1000)) e)) 0)
 
==> (1)
 
(task 'b '(a 1 (b 10 (c 100)) (b 11 (c 100 (d 1000)) e)) 1)
 
==> (10 11)
 
(task 'c '(a 1 (b 10 (c 100)) (b 11 (c 101 (d 1000)) e)) 2)
 
==> (100 101)
 
(task 'd '(a 1 (b 10 (c 100)) (b 11 (c 101 (d 1000)) e)) 3)
 
==> (1000)

(defun dive-elms (n w)
  (cond ((null w) nil)
        ((= n 0) w)
        ((atom (car w)) (dive-elms n (cdr w)))
        ((append (dive-elms (- n 1) (car w)) (dive-elms n (cdr w))))))
 
(defun next (w z)
  (cond ((null w) nil)
        ((eq (car w) z) (cons (cadr w) (next (cddr w) z)))
        ((next (cdr w) z))))
 
(defun dive-next (n w z)
  (next (dive-elms n w) z))
 
> (dive-next 0 '(a 1 (b 10 (c 100)) (b 10 (c 100 (d 1000)) e)) 'a)
(1)
> (dive-next 1 '(a 1 (b 10 (c 100)) (b 10 (c 100 (d 1000)) e)) 'b)
(10 10)
> (dive-next 2 '(a 1 (b 10 (c 100)) (b 10 (c 100 (d 1000)) e)) 'c)
(100 100)
> (dive-next 3 '(a 1 (b 10 (c 100)) (b 10 (c 100 (d 1000)) e)) 'd)
(1000)