code: public static <T> StreamEx<T> of(T element) {
          return of(new ConstSpliterator.OfRef<>(element, 1, true));
      }
docs: Returns a sequential {@code StreamEx} containing a single element.
  
  @param <T> the type of stream element
  @param element the single element
  @return a singleton sequential stream
  @see Stream#of(Object)