code: public List<ExtensionElement> getExtensions(String elementName, String namespace) {
          requireNotNullNorEmpty(elementName, "elementName must not be null or empty");
          requireNotNullNorEmpty(namespace, "namespace must not be null or empty");
          String key = XmppStringUtils.generateKey(elementName, namespace);
          return payload.getAll(key);
      }
docs: Return a list of all extensions with the given element name <em>and</em> namespace.
  <p>
  Changes to the returned set will update the stanza extensions, if the returned set is not the empty set.
  </p>
  
  @param elementName the element name, must not be null.
  @param namespace the namespace of the element(s), must not be null.
  @return a set of all matching extensions.