* identifier should be from the collection obtained by {@link #getModelIds()}. As a special case, an empty string
     * can be used as the identifier for the super POM.
     *
     * @param modelId The identifier of the desired raw model, must not be {@code null}.
     * @return The raw model or {@code null} if the specified model id does not refer to a known model.
     */
    @Nonnull
    Optional<Model> getRawModel(@Nonnull String modelId);