code: public static ExtensionElementProvider<ExtensionElement> getExtensionProvider(String elementName, String namespace) {
          String key = getKey(elementName, namespace);
          return extensionProviders.get(key);
      }
docs: Returns the stanza extension provider registered to the specified XML element name
  and namespace. For example, if a provider was registered to the element name "x" and the
  namespace "jabber:x:event", then the following stanza would trigger the provider:
  
  <pre>
  &lt;message to='romeo@montague.net' id='message_1'&gt;
  &lt;body&gt;Art thou not Romeo, and a Montague?&lt;/body&gt;
  &lt;x xmlns='jabber:x:event'&gt;
  &lt;composing/&gt;
  &lt;/x&gt;
  &lt;/message&gt;</pre>
  
  <p>Note: this method is generally only called by the internal Smack classes.
  
  @param elementName element name associated with extension provider.
  @param namespace namespace associated with extension provider.
  @return the extension provider.