code: public <R> R submit(final Function<RS, R> fn) {
          return submit(() -> fn.apply(this.results));
      }
docs: This method allows the SimpleReact Executor to be reused by JDK parallel streams. It is best used when
  collectResults and block are called explicitly for finer grained control over the blocking conditions.
  
  @param fn Function that contains parallelStream code to be executed by the SimpleReact ForkJoinPool (if configured)