title: Common lisp, undefined intersection function behaviour?
body: <p>According to CLHS entry for the INTERSECTION function (<a href="http://www.lispworks.com/documentation/HyperSpec/Body/f_isec_.htm" rel="noreferrer">http://www.lispworks.com/documentation/HyperSpec/Body/f_isec_.htm</a>):</p>
  <blockquote>
  <p>For every pair that satifies the test, <strong>exactly one of the two elements</strong>
  of the pair will be put in the result.</p>
  </blockquote>
  <p>My problem is that i need to know <strong>which one of the two elements</strong> of the pair will be put in the result, which matters when, for example  <code>:key #'car</code>  is used to extract the arguments to test against, since the cdr might be different. I would like to have a guarantee that either always the first or always the second element will be put in the result. Am I missing something or is this just unspecified behaviour, so I shouldnt use it for my case?</p>