code: @SuppressWarnings("unchecked")
      @Override
      public S parallel() {
          context = context.parallel();
          if (stream != null)
              stream = stream.parallel();
          return (S) this;
      }
docs: {@inheritDoc}
  
  <p>
  If this stream was created using {@link #parallel(ForkJoinPool)}, the new
  stream forgets about supplied custom {@link ForkJoinPool} and its
  terminal operation will be executed in common pool.