* Gets unique identifier of the model
     *
     * @return The effective identifier of the model, never {@code null}.
     */
    public String id() {
        // if source is null, it is the super model, which can be accessed via empty string
        return source != null ? source.getLocation() : "";
    }

    @Override
    public String toString() {
        return String.valueOf(model);
    }

  unimplemented, which is to detect when the network is down and provide better
  feedback for that case. However, when offline mode is declared, nothing
  earmarked as remote should be accessed, regardless of whether it is actually
  a physically local resource.

  NOTE: One side-effect of this design change is that all offline-mode code can
  be isolated to maven-core, maven-plugin-descriptor, and [possibly]