code: public <PE extends ExtensionElement> PE getExtension(String elementName, String namespace) {
          return PacketUtil.extensionElementFrom(extensions, elementName, namespace);
      }
docs: Returns the first stanza extension that matches the specified element name and
  namespace, or <tt>null</tt> if it doesn't exist.
  
  @param elementName the XML element name of the stanza extension.
  @param namespace the XML element namespace of the stanza extension.
  @param <PE> type of the ExtensionElement.
  @return the extension, or <tt>null</tt> if it doesn't exist.