* @return the root directory, never {@code null}
     * @throws IllegalStateException if the root directory could not be found
     * @see #getTopDirectory()
     * @see Project#getRootDirectory()
     * @see Project#isRootProject()
     */
    @Nonnull
    Path getRootDirectory();

    /**
     * Returns a proto session builder of this instance.
     */
    @Nonnull
    default Builder toBuilder() {
        try {

     * is {@code true} or that {@code basedir} contains a {@code .mvn} child
     * directory.
     *
     * @return {@code true} if the project is the root project
     * @see Model#isRoot()
     */
    boolean isRootProject();

    /**
     * Gets the root directory of the project, which is the parent directory
     * containing the {@code .mvn} directory or flagged with {@code root="true"}.
     *