return getProjectSelectors(pa -> !pa.optional && !pa.active);
    }

    /**
     * @return Optional inactive project selectors, never {@code null}.
     */
    public Set<String> getOptionalInactiveProjectSelectors() {
        return getProjectSelectors(pa -> pa.optional && !pa.active);
    }

    /**
     * Mimics the pre-Maven 4 "selected projects" list.

        Set<String> requiredSelectors = projectActivation.getRequiredInactiveProjectSelectors();
        Set<String> optionalSelectors = projectActivation.getOptionalInactiveProjectSelectors();
        if (!requiredSelectors.isEmpty() || !optionalSelectors.isEmpty()) {
            Set<MavenProject> excludedProjects = new HashSet<>(requiredSelectors.size() + optionalSelectors.size());

        assertThat(activation.getRequiredInactiveProjectSelectors(), containsInAnyOrder("test1", "test2"));
        assertThat(activation.getOptionalInactiveProjectSelectors(), containsInAnyOrder("test3", "test4"));

        activation = new ProjectActivation();
        performProjectActivation(parser.parse(options, new String[] {"-pl", "-test1,+test2"}), activation);