atching Directives</a></p>
<p>If the element selector is prefixed with a <code>?</code> then that slot is optional.</p>
<p>For example, the transclude object <code>{ slotA: &#39;?myCustomElement&#39; }</code> maps <code>&lt;my-custom-element&gt;</code> elements to
the <code>slotA</code> slot, which can be accessed via the <code>$transclude</code> function or via the <a href="api/ng/directive/ngTransclude"><code>ngTransclude</code></a> directive.</p>
<p>Slots that are not marked as optional (<code>?</code>) 