private static Set<String> featuresAccessedIn(ExpressionNode node) {
        Set<String> features = new HashSet<>();
        new FeatureFinder(features).process(node);
        return features;
    }
Returns this as a double binary operator

@throws IllegalStateException if this has more than two arguments
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/LambdaFunctionNode.java