* equal.
   */
  boolean areEqual();

  /**
   * Returns an unmodifiable map containing the entries from the left map whose keys are not present
   * in the right map.
   */
  Map<K, V> entriesOnlyOnLeft();

  /**
   * Returns an unmodifiable map containing the entries from the right map whose keys are not
   * present in the left map.
   */
  Map<K, V> entriesOnlyOnRight();

  /**

    /**
     * Merges values from the specified parent model into the given child model. Implementations are expected to keep
     * parent and child completely decoupled by injecting deep copies of objects into the child rather than the original
     * objects from the parent.
     *
     * @param child The child model into which to merge the values inherited from the parent, must not be
     *            <code>null</code>.

    private void visit(MetadataGraphVertex from, List<MetadataGraphVertex> visited, MetadataGraph graph) {
        if (visited.contains(from)) {
            return;
        }

        visited.add(from);

        List<MetadataGraphEdge> exitList = graph.getExcidentEdges(from);
        // String s = "|---> "+from.getMd().toString()+" - "+(exitList == null ? -1 : exitList.size()) + " exit links";

        } catch (FileNotFoundException e) {
            throw new RepositoryMetadataReadException("Cannot read metadata from '" + mappingFile + "'", e);
        } catch (IOException | XMLStreamException e) {
            throw new RepositoryMetadataReadException(
                    "Cannot read metadata from '" + mappingFile + "': " + e.getMessage(), e);
        }
    }

    /**

    assertNoTypeVariable(new From<Integer>().new To<String>() {}.type());
    assertHasTypeVariable(new From<Integer>().new To<T>() {}.type());
    assertHasTypeVariable(new From<T>().new To<String>() {}.type());

    // both subclassed
    assertHasTypeVariable(new From<T>() {}.new To<String>() {}.type());
    assertNoTypeVariable(new From<Integer>() {}.new To<String>() {}.type());

     *            be {@code null}.
     * @param session The build session from which to pick the current project and repository settings, must not be
     *            {@code null}.
     * @param parent The parent class realm for the plugin, may be {@code null} to use the Maven core realm.
     * @param imports The packages/types to import from the parent realm, may be {@code null}.

    }

    @Override
    public List<Toolchain> getToolchains(Session session, String type, Map<String, String> requirements)
            throws ToolchainManagerException {
        MavenSession s = InternalMavenSession.from(session).getMavenSession();
        List<org.apache.maven.toolchain.Toolchain> toolchains =
                toolchainManagerPrivate.getToolchains(s, type, requirements);
        return new MappedList<>(toolchains, this::toToolchain);

     */
    String getArtifactId();

    /**
     * The repository from which the plugin prefix was resolved.
     *
     * @return The repository from which the plugin prefix was resolved or {@code null} if the prefix was resolved from
     *         the supplied POM.
     */
    ArtifactRepository getRepository();

     * 
     * @param to
     * @param from
     */
    public static void cloneInternal ( Kerb5Authenticator to, Kerb5Authenticator from ) {
        NtlmPasswordAuthenticator.cloneInternal(to, from);
        to.setUser(from.getUser());
        to.setRealm(from.getRealm());
        to.setService(from.getService());
        to.setLifeTime(from.getLifeTime());
        to.setUserLifeTime(from.getUserLifeTime());

  static final String NODE_REMOVED_FROM_GRAPH =
      "Node %s that was used to generate this set is no longer in the graph.";
  static final String NODE_PAIR_REMOVED_FROM_GRAPH =
      "Node %s or node %s that were used to generate this set are no longer in the graph.";
  static final String EDGE_REMOVED_FROM_GRAPH =
      "Edge %s that was used to generate this set is no longer in the graph.";