title: Is there a way to print an error message when you try to print a variable?
body: <p>For example:</p>
  
  <pre><code>CL-USER 1 &gt; (setq l1 '(a b c))
  (A B C)
  
  CL-USER 2 &gt; l1
  (A B C)
  </code></pre>
  
  <p>When I type <strong><em>l1</em></strong>, instead of <strong><em>(A B C)</em></strong> I want to print a different message, is that possible? Because is not a manipulation of a function, where I put a condition and instead of the list I print a message.. In this case there's no function.</p>