title: Why is the @ sign needed in this macro definition?
body: <p>In the following <code>when</code> macro:</p>
  <pre><code>(defmacro when (condition &amp;rest body)
    `(if ,condition (progn ,@body)))
  </code></pre>
  <p>Why is  there an &quot;at&quot; <code>@</code> sign?</p>