private String lookup(String expressionName, Map<String, String> ranking) {
        String value = ranking.get("rankingExpression(" + expressionName + ").rankingScript");
        if (value == null) {
            return expressionName;
        }
        return value;
    }
Recursively resolves references to other ranking expressions - rankingExpression(name) -
and replaces the reference by the expression
config-model/src/test/java/com/yahoo/schema/derived/GeminiTestCase.java