title: Less and Bootstrap: how to use a span3 (or spanX [any number]) class as a mixin?
body: <p>Is it possibile to add <code>span3</code> class in a mixin to avoid putting it in every element in my HTML?
  Something like:</p>
  
  <pre><code>.myclass {
      .span3;
      // other rules...
  }
  </code></pre>
  
  <p><strong>EDIT</strong></p>
  
  <p>I apologize I forgot to specify an important detail: <code>span3</code> is a standard class of Bootstrap. 
  I didn't find its definition in any file of the Bootstrap framework. </p>