title: Survey: how are you using ID/IDREF? (or key/keyref)
body: <p>I'm interested in how people use this feature: what domain, what relationships (apologies if survey questions aren't appropriate).</p>
  
  <p>XML represents trees naturally using containment; but it can also represent graphs, using IDREFs that refer to IDs: many elements can refer to the same element (multi-ref); and a nested element can refer to an ancestor (cycle).  It might also be convenient to use IDs and IDREFs so the XML structure matches another storage format, e.g. database foreign keys.</p>
  
  <p>What do <em>you</em> represent with ID/IDREF?</p>
  
  <p><em>EDIT</em> Also including <code>&lt;key&gt;/&lt;keyref&gt;</code> from XML Schema.</p>