runtime prior to use.
 <p>
 Implementations should warn about and ignore classes that do not conform to the requirements of root resource or
 provider classes. Implementations should flag an error if the returned set includes more than one instance of the
 same class. 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 instances. Returning <co