title: jquery prevent multi selection in multi-select
body: <p>I everybody,</p>
  
  <p>I've a multi-select like this:</p>
  
  <pre><code>&lt;select multiple="multiple" id="lista"&gt;
      &lt;option value="a"&gt;Mario&lt;/option&gt;
      &lt;option value="b"&gt;Maria&lt;/option&gt;
      &lt;option value="c"&gt;Mark&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>How can I prevent to select multiple option by a user? (<em>shift+arrow up/down</em>)</p>
  
  <p>thanks in advance</p>
  
  <p>ciao
  h.</p>