import org.apache.maven.api.annotations.Nonnull;

/**
 * Transporter provider is a service that provides somewhat trivial transport capabilities backed by Maven internals.
 * This API does not try to cover all the requirements out there, just the basic ones, and is intentionally simple.
 * If plugin or extension needs anything more complex feature wise (i.e. HTTP range support or alike) it should
 * probably roll its own.
 * <p>

    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) {

    }

    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.
     */

    }

    private void assertVersionOlder(String left, String right) {
        assertTrue(
                newArtifactVersion(left).compareTo(newArtifactVersion(right)) < 0,
                left + " should be older than " + right);
        assertTrue(
                newArtifactVersion(right).compareTo(newArtifactVersion(left)) > 0,
                right + " should be newer than " + left);
    }