code: @SequentialOnly
      public static <T> Function<T, Indexed<T>> indexed() {
          return new Function<T, Indexed<T>>() {
              private final MutableLong idx = new MutableLong(0);
  
              @Override
              public Indexed<T> apply(T t) {
                  return Indexed.of(t, idx.getAndIncrement());
              }
          };
      }
docs: Returns a stateful <code>Function</code> which should not be used in parallel stream.
  
  @return