.filter(p -> !request.getPluginGroups().contains(p.getGroupId()))
                .collect(Collectors.groupingBy(
                        Plugin::getGroupId,
                        LinkedHashMap::new,
                        Collectors.mapping(Plugin::getArtifactId, Collectors.toSet())));
        request.getPluginGroups().forEach(g -> candidates.put(g, null));

        }

        return this;
    }

    @Override
    public MavenExecutionRequest addPluginGroup(String pluginGroup) {
        if (!getPluginGroups().contains(pluginGroup)) {
            getPluginGroups().add(pluginGroup);
        }

        return this;
    }

    @Override
    public MavenExecutionRequest addPluginGroups(List<String> pluginGroups) {

    public ProjectBuildingRequest getProjectBuildingRequest() {
        return request.getProjectBuildingRequest().setRepositorySession(getRepositorySession());
    }

    public List<String> getPluginGroups() {
        return request.getPluginGroups();
    }

    public boolean isOffline() {
        return request.isOffline();
    }

    public MavenProject getTopLevelProject() {
        return topLevelProject;
    }

            return request;
        }

        request.setOffline(settings.isOffline());

        request.setInteractiveMode(settings.isInteractiveMode());

        request.setPluginGroups(settings.getPluginGroups());

        request.setLocalRepositoryPath(settings.getLocalRepository());

        for (Server server : settings.getServers()) {
            request.addServer(server);
        }

        if (settings.isUsePluginRegistry()) {
            addViolation(problems, Severity.WARNING, "usePluginRegistry", null, "is deprecated and has no effect.");
        }

        List<String> pluginGroups = settings.getPluginGroups();

        if (pluginGroups != null) {
            for (int i = 0; i < pluginGroups.size(); i++) {
                String pluginGroup = pluginGroups.get(i).trim();

                if (pluginGroup.isEmpty()) {

    // Mirrors
    List<Mirror> getMirrors();

    MavenExecutionRequest setMirrors(List<Mirror> mirrors);

    MavenExecutionRequest addMirror(Mirror mirror);

    // Plugin groups
    List<String> getPluginGroups();

    MavenExecutionRequest setPluginGroups(List<String> pluginGroups);

    MavenExecutionRequest addPluginGroup(String pluginGroup);

    MavenExecutionRequest addPluginGroups(List<String> pluginGroups);

            return;
        }
        request.setOffline(settings.isOffline());
        request.setInteractiveMode(settings.isInteractiveMode());
        request.setPluginGroups(settings.getPluginGroups());
        request.setLocalRepositoryPath(settings.getLocalRepository());
        for (Server server : settings.getServers()) {
            request.addServer(new org.apache.maven.settings.Server(server));
        }