return checkNotNull(iterator);
  }

  /**
   * Returns an iterator over the merged contents of all given {@code iterators}, traversing every
   * element of the input iterators. Equivalent entries will not be de-duplicated.
   *
   * <p>Callers must ensure that the source {@code iterators} are in non-descending order as this
   * method does not sort its input.
   *