String toRankingExpression() {
        StringBuilder ret = new StringBuilder();
        for (int i = 0; i < xgboostTrees.size(); i++) {
            ret.append(treeToRankExp(xgboostTrees.get(i)));
            if (i != xgboostTrees.size() - 1) {
                ret.append(" + \n");
            }
        }
        return ret.toString();
    }
Converts parsed JSON trees to Vespa ranking expressions.

@return Vespa ranking expressions.
model-integration/src/main/java/ai/vespa/rankingexpression/importer/xgboost/XGBoostParser.java