anchor.diff:          (t
            list-1)))
   
  +(defun nset-difference (list-1 list-2 &key key test test-not)
  +  (set-difference list-1 list-2 :key key :test test :test-not test-not))
  +
   (defun union (list-1 list-2 &key key test test-not)
     (when (not key)
       (setf key 'identity))
  
positive.code:       (when (not (member (funcall key e) list-1 :key key :test test :test-not test-not))
          (push e result)))
      result))
  
  (defun subsetp (list-1 list-2 &key key test test-not)
    (when (not key)
  
negative.code:              alist
               :key key))
  
  (declaim (inline set-difference nset-difference
                   union nunion
                   intersection nintersection
                   set-exclusive-or nset-exclusive-or))
  
  (defun set-difference (list-1 list-2 &key key test test-not)
    (when (not key)
      (setf key 'identity))
  