* (prefixed with {@code env.}).
     *
     * @return the system properties, never {@code null}
     */
    @Nonnull
    Map<String, String> getSystemProperties();

    /**
     * Returns the properly overlaid map of properties: system + user.
     */
    @Nonnull
    Map<String, String> getEffectiveProperties();

    /**
     * Returns the start time of the session.
     *

    /**
     * Gets the type of the event.
     *
     * @return the type of the event, never {@code null}
     */
    @Nonnull
    EventType getType();

    /**
     * Gets the session from which this event originates.
     *
     * @return the current session, never {@code null}
     */
    @Nonnull
    Session getSession();

    /**
     * Gets the current project (if any).
     *

                    Arrays.spliterator(new String[] {"abc", "", "de", "f", "g", ""}),
                    (String str) -> charactersOf(str).spliterator(),
                    Spliterator.SIZED | Spliterator.DISTINCT | Spliterator.NONNULL,
                    7))
        .expect('a', 'b', 'c', 'd', 'e', 'f', 'g');
  }

  public void testFlatMap_nullStream() {
    SpliteratorTester.of(
            () ->
                CollectSpliterators.flatMap(

     * @throws IllegalArgumentException if an argument is {@code null} or invalid
     * @see #build(ToolchainsBuilderRequest)
     */
    @Nonnull
    default ToolchainsBuilderResult build(
            @Nonnull Session session,
            @Nonnull Source installationToolchainsFile,
            @Nonnull Source userToolchainsSource) {
        return build(ToolchainsBuilderRequest.build(session, installationToolchainsFile, userToolchainsSource));
    }

     *
     * @return the child nodes of this node, never {@code null}
     */
    @Nonnull
    List<Node> getChildren();

    /**
     * @return repositories of this node
     */
    @Nonnull
    List<RemoteRepository> getRemoteRepositories();

    /**
     * The repository where this artifact has been downloaded from.
     */
    @Nonnull
    Optional<RemoteRepository> getRepository();

    /**

    /**
     * The packaging id.
     */
    @Nonnull
    @Override
    String id();

    /**
     * The language of this packaging.
     */
    @Nonnull
    default Language language() {
        return type().getLanguage();
    }

    /**
     * The type of main artifact produced by this packaging.
     */
    @Nonnull
    Type type();

    /**

     *
     * @throws TransportProviderException if passed in remote repository has invalid remote URL or unsupported protocol.
     */
    @Nonnull
    Transport transport(@Nonnull Session session, @Nonnull RemoteRepository repository);

     *
     * @return the operating system name (never null)
     */
    @Nonnull
    String name();

    /**
     * Returns the OS architecture as reported by the system property "os.arch".
     * The value is converted to lowercase for consistency.
     *
     * @return the operating system architecture (never null)
     */
    @Nonnull
    String arch();

    /**

     */
    <T> boolean replace(@Nonnull Key<T> key, @Nullable T oldValue, @Nullable T newValue);

    /**
     * Gets the session data associated with the specified key.
     *
     * @param key the key for which to retrieve the session data, must not be {@code null}
     * @return the session data associated with the key or {@code null} if none
     */
    @Nullable
    <T> T get(@Nonnull Key<T> key);

    /**

     * @param source the source of the project to build
     * @return a new ProjectBuilderRequest
     * @throws NullPointerException if session or source is null
     */
    @Nonnull
    static ProjectBuilderRequest build(@Nonnull Session session, @Nonnull Source source) {
        return builder()
                .session(requireNonNull(session, "session cannot be null"))
                .source(requireNonNull(source, "source cannot be null"))