code: @SuppressWarnings("rawtypes")
      public static <T> Collector<T, ?, Optional<T>> onlyOne() {
          final Supplier<Holder<Optional<T>>> supplier = (Supplier) onlyOne_supplier;
          final BiConsumer<Holder<Optional<T>>, T> accumulator = (BiConsumer) onlyOne_accumulator;
          final BinaryOperator<Holder<Optional<T>>> combiner = (BinaryOperator) onlyOne_combiner;
          final Function<Holder<Optional<T>>, Optional<T>> finisher = (Function) onlyOne_finisher;
  
          return new CollectorImpl<>(supplier, accumulator, combiner, finisher, CH_UNORDERED_NOID);
      }
docs: {@code DuplicatedResultException} is threw if there are more than one values are collected.
  
  @return