public File relativeFile(String relativePath, String descriptionOfPath) {
        File file = new File(new File(source()).getParent(), relativePath);
        if ( ! file.exists())
            throw new IllegalArgumentException(descriptionOfPath + " references '" + relativePath +
                                               "', but this file does not exist");
        return file;
    }
Returns a reference to the File at a path given relative to the source root of this model

@throws IllegalArgumentException if the path is illegal or non-existent
model-integration/src/main/java/ai/vespa/rankingexpression/importer/ImportedModel.java