(3rd y 2nd)) ;3rd follows 2nd down the list.
((atom 2nd) 3rd)
(rplacd 2nd 3rd)))
+
+
+(defun adjoin (item list &key (test #'eql) (key #'identity))
+ (if (member item list :key key :test test)
+ list
+ (cons item list)))
+
+(defun intersection (list1 list2 &key (test #'eql) (key #'identity))
+ (let ((new-list ()))
+ (dolist (x list1)
+ (when (member x list2 :test test :key key)
+ (push x new-list)))
+ new-list))