code: @CheckReturnValue
      public Maybe<E> maybe() {
          return Maybe.fromCallable(new Callable<E>() {
              @Override
              public E call() throws Exception {
                  return firstOrNull();
              }
          });
      }
docs: Converts the result stream to a {@link io.reactivex.Maybe} value, return the first element
  if present or completes if no results.
  
  @return maybe instance of the results of this query.