code: public Stream<Indexed<T>> index() {
          return map(new Func1<T, Indexed<T>>() {
  
              int i;
  
              @Override
              public Indexed<T> call(T value) {
                  return new Indexed<>(i++, value);
              }
          });
      }
docs: Returns a new stream of {@link Indexed} that where each item's index is equal to its sequence number.
  
  @return a new stream of {@link Indexed} that where each item's index is equal to its sequence number.