code: public static DocPath forRoot(PackageDoc pd) {
          String name = (pd == null) ? "" : pd.name();
          if (name.isEmpty())
              return empty;
          return new DocPath(name.replace('.', '/').replaceAll("[^/]+", ".."));
      }
docs: Return the inverse path for a package.
  For example, if the package is java.lang,
  the inverse path is ../...