code: public <P_IN> Node<E_OUT> opEvaluateParallel(PipelineHelper<E_OUT> helper,
                                            Spliterator<P_IN> spliterator,
                                            IntFunction<E_OUT[]> generator) {
          throw new UnsupportedOperationException("Parallel evaluation is not supported");
      }
docs: Performs a parallel evaluation of the operation using the specified
  {@code PipelineHelper} which describes the upstream intermediate
  operations.  Only called on stateful operations.  If {@link
  #opIsStateful()} returns true then implementations must override the
  default implementation.
  
  @implSpec The default implementation always throw
  {@code UnsupportedOperationException}.
  
  @param helper the pipeline helper describing the pipeline stages
  @param spliterator the source {@code Spliterator}
  @param generator the array generator
  @return a {@code Node} describing the result of the evaluation