private ExpressionNode expandRRF(Reference ref) {
        var args = ref.arguments();
        if (args.size() < 2) {
            throw new IllegalArgumentException("must have at least 2 arguments: " + ref);
        }
        List<ExpressionNode> children = new ArrayList<>();
        List<Operator> operators = new ArrayList<>();
        for (var arg : args.expressions()) {
            if (! children.isEmpty()) operators.add(Operator.plus);
            children.add(new ReferenceNode(RECIPROCAL_RANK, List.of(arg), null));
        }
        // must be further transformed (see above)
        return new OperationNode(children, operators);
    }
Recognizes pseudo-functions and creates global-phase normalizers
@author arnej
config-model/src/main/java/com/yahoo/schema/expressiontransforms/NormalizerFunctionExpander.java