}

    public List<Extension> getBuildExtensions() {
        Build build = getBuild();
        if ((build == null) || (build.getExtensions() == null)) {
            return Collections.emptyList();
        } else {
            return Collections.unmodifiableList(build.getExtensions());
        }
    }

    public void addProjectReference(MavenProject project) {
        projectReferences.put(

            // extensionArtifacts
            Set<Artifact> extensionArtifacts = new HashSet<>();
            List<Extension> extensions =
                    project.getModel().getDelegate().getBuild().getExtensions();
            if (extensions != null) {
                for (Extension ext : extensions) {
                    String version;
                    if (ext.getVersion() == null || ext.getVersion().isEmpty()) {

                            EMPTY,
                            problems,
                            errOn30,
                            Version.V20,
                            p.getExtensions(),
                            p.getKey(),
                            p);

                    validate20EffectivePluginDependencies(problems, p, request);
                }