checkState(hasParent(), "Domain '%s' has no parent", name);
    return ancestor(1);
  }

  /**
   * Returns the ancestor of the current domain at the given number of levels "higher" (rightward)
   * in the subdomain list. The number of levels must be non-negative, and less than {@code N-1},
   * where {@code N} is the number of parts in the domain.
   *