final Map<Path, Holder> modelByPath = new ConcurrentHashMap<>();

    final Map<GAKey, Holder> modelByGA = new ConcurrentHashMap<>();

    public static class Holder {
        private volatile boolean set;
        private volatile Model model;

        Holder() {}

        Holder(Model model) {
            this.model = Objects.requireNonNull(model);

                Model model = findRawModel(from, gId, aId);
                if (model != null) {
                    context.modelByGA.put(new GAKey(gId, aId), new Holder(model));
                    context.modelByPath.put(model.getPomFile(), new Holder(model));
                }
                return model;
            }

            @Override
            public Model getRawModel(Path from, Path path) {

    }

    private static final long serialVersionUID = 5;
  }

  public void testMutableValues() {
    IntHolder holderA = new IntHolder(1);
    IntHolder holderB = new IntHolder(2);
    Map<String, IntHolder> map = ImmutableMap.of("a", holderA, "b", holderB);
    holderA.value = 3;
    assertTrue(map.entrySet().contains(Maps.immutableEntry("a", new IntHolder(3))));

            a.attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category.class, Category.DOCUMENTATION));
            a.attribute(DocsType.DOCS_TYPE_ATTRIBUTE, objects.named(DocsType.class, "gradle-source-folders"));
        });
        sourcesPath.setCanBeConsumed(false);
        sourcesPath.setCanBeResolved(true);
        sourcesPath.setVisible(false);
        sourcesPath.extendsFrom(runtimeClasspath);

        OrArtifactFilter filter2 = new OrArtifactFilter(Arrays.asList(newSubFilter()));

        assertFalse(filter1.equals(null));
        assertTrue(filter1.equals(filter1));
        assertEquals(filter1.hashCode(), filter1.hashCode());

        assertFalse(filter1.equals(filter2));
        assertFalse(filter2.equals(filter1));
    }

    }

    protected DefaultModelRepositoryHolder(DefaultModelRepositoryHolder holder) {
        this.session = holder.session;
        this.repositoryMerging = holder.repositoryMerging;
        this.pomRepositories = List.copyOf(holder.pomRepositories);
        this.externalRepositories = List.copyOf(holder.externalRepositories);
        this.repositories = List.copyOf(holder.repositories);
    }

    @Override

     *
     * @param session The session to use to resolve the model, must not be {@code null}.
     * @param parent The parent coordinates to resolve, must not be {@code null}.
     * @param modified a holder for the updated parent, must not be {@code null}.
     * @return The source of the requested POM, never {@code null}.
     * @throws ModelResolverException If the POM could not be resolved from any configured repository.
     */

                    Version.V20,
                    "modelVersion",
                    null,
                    "of '" + string + "' is older than the versions supported by this version of Maven: "
                            + validVersions + ". Building this project requires an older version of Maven.",
                    tracker);

        } else {
            addViolation(
                    problems,