3.1 union, intersection

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(defun union (a b &key (test #'eq))
  (cond
   ((null a) b)
   ((member (car a) b :test test)
    (union (cdr a) b :test test))
   (t (cons (car a) (union (cdr a) b :test test)))))
(defun intersection (a b &key (test #'eq))
  (cond
   ((null a) nil)
   ((member (car a) b :test test)
    (cons (car a) (intersection (cdr a) b :test test)))
   (t (intersection (cdr a) b :test test))))