will trigger a compile time error if there are no matching elements
in the transclude content. If you wish to know if an optional slot was filled with content, then you can call
<code>$transclude.isSlotFilled(slotName)</code> on the transclude function passed to the directive&#39;s link function and
injectable into the directive&#39;s controller.</p>
<h4 id="transclusion-functions">Transclusion Functions</h4>
<p>When a directive requests transclusion, the compiler extracts its contents and provides a <stron