sts()</code>)<br/><span style="color:DarkGray;text-decoration:line-through"><b title="Formal Invariant Identifier">ext-1</b>: Must have either extensions or value[x], not both (extension.exists() != value.exists())</span></td></tr>

<tr><td colspan="2" class="structure"><span class="self-link-parent"><a name="key_Extension.extension">  </a><span style="color: grey">2</span><b>. Extension.extension</b><a href="#key_Extension.extension" title="link to here" class="self-link"><svg xmlns="http://www.w3.org/20