code: protected URL getResourceURLOrSystemId() throws Exception {
          URL url = getResourceURL();
          if (url != null) return url;
          return new URL(getResolvedSystemId());
      }
docs: Return the {@linkplain #getResourceURL() resource URL},
  or the  {@linkplain #getResolvedSystemId() system ID} as URL.
  @return url
  @throws Exception