title: Graphviz Dot, mix directed and undirected
body: <p>For my application I need to represent simultaneously (on the same graph) two relations: one is simmetric, the other is not.</p>
  
  <p>Targets:</p>
  
  <ul>
  <li>Ideally the two relation should result in edges having different colors;</li>
  <li>For the symmetric relation I would like not to have double-edges;</li>
  </ul>
  
  <p>Is there a way of doing this with <em>dot</em>?</p>