hould warn about and ignore classes that do not conform to the requirements of root resource or
 provider/feature classes. Implementations should warn about and ignore classes for which <a href="#getSingletons()"><code>getSingletons()</code></a>
 returns an instance. Implementations MUST NOT modify the returned set.
 </p>
 <p>
 The default implementation returns an empty set.
 </p></div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a set of root resource and provider classes. Returning <code>