code: public static Predicate<HandlerInput> selectedElementToken(String elementToken) {
          return i -> i.getRequestEnvelope().getRequest() instanceof ElementSelectedRequest
                  && elementToken.equals(((ElementSelectedRequest) i.getRequestEnvelope().getRequest()).getToken());
      }
docs: Returns a predicate that returns to true if the incoming request is an {@link ElementSelectedRequest}
  with the given token.
  @param elementToken token to evaluate against
  @return true if the incoming request is an {@link ElementSelectedRequest} with the given token