title: Common lisp :KEY parameter use
body: <p>The <code>:KEY</code> parameter is included in some functions that ship with Common Lisp. All of the descriptions that I have found of them are unhelpful, and <code>:KEY</code> is difficult to search in a search engine because the ":" is usually ignored.</p>
  
  <p>How would it be used, for example, in the <code>member</code> function which allows both <code>:TEST</code> and <code>:KEY</code>?</p>