code: private InternetDomainName ancestor(int levels) {
      return from(DOT_JOINER.join(parts.subList(levels, parts.size())));
    }
docs: 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.
  
  <p>TODO: Reasonable candidate for addition to public API.