*
     * So {@code (b : c).plus(a) == (a : b : c)}
     */
    @CheckReturnValue
    public abstract PersistentList<T> plus(T element);

    public abstract boolean isEmpty();

    private PersistentList() {}

    private static final PersistentList<Object> NIL = new PersistentList<Object>() {
        @Override
        public void forEach(Consumer<? super Object> consumer) {
        }

            // The file pom may not contain the modelVersion yet, as it may be set later by the
            // ModelVersionXMLFilter.
            if (m.getModelVersion() != null && !m.getModelVersion().isEmpty()) {
                validateModelVersion(problems, m.getModelVersion(), m, VALID_MODEL_VERSIONS);
            }

            validateStringNoExpression("groupId", problems, Severity.WARNING, Version.V20, m.getGroupId(), m);

            .collect(Collectors.toList());

        StyledTextOutput styledTextOutput = getRenderer().getTextOutput();
        if (!softwareTypes.isEmpty()) {
            renderSectionTitle("Available software types");
            styledTextOutput.println();

            softwareTypes.forEach(type -> {

        }
    }

    @Override
    public synchronized Throwable getCause() {
        if (hideCause.get()) {
            return null;
        }
        return causes.isEmpty() ? null : causes.get(0);
    }

    @SuppressWarnings("DefaultCharset")
    @Override
    public void printStackTrace(PrintStream printStream) {
        PrintWriter writer = new PrintWriter(printStream);

                if (dependency.getGroupId() == null
                        || dependency.getGroupId().isEmpty()
                        || dependency.getArtifactId() == null
                        || dependency.getArtifactId().isEmpty()
                        || dependency.getVersion() == null
                        || dependency.getVersion().isEmpty()) {
                    // guard against case where best-effort resolution for invalid models is requested

                profileSelector.getActiveProfiles(request.getProfiles(), profileActivationContext, problems);

        result.setActiveExternalProfiles(activeExternalProfiles);

        if (!activeExternalProfiles.isEmpty()) {
            Properties profileProps = new Properties();
            for (Profile profile : activeExternalProfiles) {
                profileProps.putAll(profile.getProperties());
            }

            Model target,
            List<? extends DependencyManagement> sources,
            ModelBuildingRequest request,
            ModelProblemCollector problems) {
        if (sources != null && !sources.isEmpty()) {
            Map<String, Dependency> dependencies = new LinkedHashMap<>();

            DependencyManagement depMgmt = target.getDependencyManagement();

            if (depMgmt != null) {

            }
        }
        assertTrue(value, !result.isEmpty() && result.get(0) > 0);
        return result;
    }

    private static boolean isNumberStartingWithZero(String number) {
        return number.length() > 1 && number.startsWith("0");
    }

    private static int findFirstNonVersionCharIndex(String s) {
        assertTrue(s, !s.isEmpty());

        for (int i = 0; i < s.length(); ++i) {

        values.add(0, new ScopeState());
    }

    protected ScopeState getScopeState() {
        if (values.isEmpty()) {
            throw new OutOfScopeException("Cannot access session scope outside of a scoping block");
        }
        return values.get(0);
    }

    public void exit() {
        if (values.isEmpty()) {
            throw new IllegalStateException();
        }
        values.remove(0);
    }

        // Do not share an ArtifactSet if the artifacts are modified by the dependency.
        if (!dependency.getDependencyMetadata().getArtifacts().isEmpty() ||
            !dependency.getAttributes().isEmpty() ||
            dependency.getExclusions().mayExcludeArtifacts()
        ) {
            int id = nextId++;