code: public static SimpleReact parallelBuilder(final int parallelism) {
          return SimpleReact.builder()
                            .executor(new ForkJoinPool(
                                                       parallelism))
                            .async(true)
                            .build();
      }
docs: Construct a new SimpleReact builder, with a new task executor and retry executor
  with configured number of threads
  
  @param parallelism Number of threads task executor should have
  @return eager SimpleReact instance