Удалите перед каждым вхождением X один элемент, если он отличен от X

(defun ante-del (a w)
  (cond ((null w) nil)
        ((and (equalp (cadr w) a) (not (equalp (car w) a))) (ante-del a (cdr w)))
        ((cons (car w) (ante-del a (cdr w))))))
 
> (ante-del 7 '(7 2 7 7 3))
(7 7 7 3)

(defun ante-del (a w)
  (when w (if (and (equalp (cadr w) a) (not (equalp (car w) a)))
              (ante-del a (cdr w))
              (cons (car w) (ante-del a (cdr w))))))
 
> (ante-del 7 '(7 2 7 7 3))
(7 7 7 3)

(defun task (lst w)
  (let ((r nil)
        (p (car lst)))
       (iter (for c in (cdr lst))  
             (when (or (Not (eq c w)) (and (eq c w) (eq p w))) (collecting p into r))
             (setq p c)) 
       (append r (list p)))) 
 
==> task
 
(task '(7 2 7 7 3) 7)
 
==> (7 7 7 3)
 
(task '(7 2 7 1 7 3) 7)
 
==> (7 7 7 3)

(defun task (lst w)
  (let ((r nil)
        (p (list (car lst))))
       (iter (for c in (cdr lst))  
             (if (or (Not (eq c w)) (and (eq c w) (eq (pop p) w))) (collecting (car p) into r) (pop p))
             (push c p)) 
       (append r (list (pop p))))) 
 
==> task
 
(task '(7 2 7 1 7 3) 7)
 
==> (7 7 7 3)
 
(task '(7 2 7 1 7 3 7) 7)
 
==> (7 7 7 7)

(defun del-before (k lst)
 (prog (acc)
    (cond ((null lst) (go c)))
  a (cond 
      ((and (eq k (cadr lst))
            (not (eq k (car lst)))) (setq lst (cdr lst))))
    (setq acc (append acc (list (car lst))))
    (setq lst (cdr lst))
  c (cond ((null lst) (return acc))
          (t (go a)))))
 
==> del-before
 
(del-before 3 '(3 1 2 3 3 4 3 3 3 5 6 3))
 
==> (3 1 3 3 3 3 3 5 3)