@Contract(pure = true)
  public static @NotNull <T> Iterator<T> concatIterators(@NotNull Collection<? extends Iterator<? extends T>> iterators) {
    return new SequenceIterator<>(iterators);
  }
@Contract(pure = true)
  public static @NotNull <T> Iterator<T> concatIterators(@NotNull Collection<? extends Iterator<? extends T>> iterators)