title: Incomplete to difference lists
body: <p>I want to convert incomplete lists into difference lists and vice versa.</p>
  
  <p>This is code to convert regular list to difference:</p>
  
  <pre><code>reg2diff(L,X-Y):-append(L,Y,X).
  </code></pre>
  
  <p>How do I go the other direction?</p>