Найти пересечение двух множеств чисел

(defun foo (elem1 lst)
    (if (= elem1 (car lst))
        t
        (if (null (cdr lst))  
            nil
            (foo elem1 (cdr lst))
        )
    )
)
 
(defun bar (lst1 lst2)
    (labels
        ((iddqd (lst1 lst2 &optional (ls nil))
            (if (foo (car lst1) lst2)
                (if (null (cdr lst1))
                    (progn
                        (if (foo (car lst1) lst2)
                            (cons (car lst1) ls)
                        )
                        (list (reverse ls))
                    )
                    (iddqd (cdr lst1) lst2 (cons (car lst1) ls))
                )
                (if (null (cdr lst1))
                    (list (reverse ls))
                    (iddqd (cdr lst1) lst2 ls)
                )
            )
        ))
        (iddqd lst1 lst2)
    )
)
 
(bar '(1 2 3 9 8) '(3 2 1 9 8 8)) ;вызов

(defun intersect (lst1 lst2)
  (cond ((null lst1) nil)
        ((member (car lst1) lst2) (cons (car lst1) (intersect (cdr lst1) lst2)))
        (t (intersect (cdr lst1) lst2))))
 
==> INTERSECT
 
(intersect '(1 2 3 4 5) '(3 4 5 6 7))
 
==> (3 4 5)

(defun _intersection (w v &optional acc &aux (a (car w)))
  (cond ((null w) (nreverse acc))
        ((member a v) (_intersection (cdr w) v (cons a acc )))
        ((_intersection (cdr w) v acc))))
 
> (_intersection '(a s d f g h ) '(q a w s e d r))
(A S D)

(defun _intersection (w v)
  (cond ((null w) nil)
        ((member (car w) v) (cons (car w) (_intersection (cdr w) v)))
        ((_intersection (cdr w) v))))
 
> (_intersection '(a s d f g h ) '(q a w s e d r))
(A S D)

(defun _intersection (w v &aux (a (car w)))
  (when w (if (member a v)
              (cons a (_intersection (cdr w) v))
              (_intersection (cdr w) v))))
 
> (_intersection '(a s d f g h ) '(q a w s e d r))
(A S D)

(defun _intersection (w v) 
  (loop for a in w when (member a v) collect a))
 
> (_intersection '(a s d f g h ) '(q a w s e d r))
(A S D)

(defun _intersection (w v)
  (remove-if-not #'(lambda (a) (member a v)) w)) 
 
> (_intersection '(a s d f g h ) '(q a w s e d r))
(A S D)

(defun _intersection (w v) (intersection w v))
 
> (_intersection '(a s d f g h ) '(q a w s e d r))
(A S D)