) &gt; id.count())</code>)<br/><b title="Formal Invariant Identifier">ext-1</b>: Must have either extensions or value[x], not both (<code>extension.exists() != value.exists()</code>)</td></tr>

<tr><td colspan="2" class="structure"><span class="self-link-parent"><a name="Extension.id">  </a><span style="color: grey">2</span><b>. Extension.id</b><a href="#Extension.id" title="link to here" class="self-link"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1792 1792" xmlns:xlink="http://www.w3.org/1999/