寫一個保留原本列表中元素順序的 union 版本：

> (new-union '(a b c) '(b a d))
(A B C D)

CL-USER> (defun our-union (fi la)
       (and (listp fi)
       (listp la)
       (let (( res (reverse fi)))
             (dolist (elt la)
               (pushnew elt res))
             (reverse res))))
STYLE-WARNING: redefining COMMON-LISP-USER::OUR-UNION in DEFUN
OUR-UNION
CL-USER> (our-union '(a b c) '(b a d))
(A B C D)
CL-USER> (our-union 'a '(b a d))
NIL
CL-USER> (our-union '(a b) 'a)
NIL
CL-USER> (our-union '(a b) 'a)
NIL

lisp 習題 our-union

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