public StringBuilder toString(StringBuilder b, SerializationContext context, Deque<String> path, CompositeNode parent) {
        b.append(name());
        if (!arguments.expressions().isEmpty()) {
            b.append("(");
            if (isSimpleRankingExpressionWrapper()) {
                b.append(simpleArgument().get());
            } else {
                for (int i = 0; i < arguments.expressions().size(); i++) {
                    ExpressionNode e = arguments.expressions().get(i);
                    e.toString(b, context, path, parent);
                    if (i+1 < arguments.expressions().size()) {
                        b.append(',');
                    }
                }
            }
            b.append(")");
        }
        if (output != null)
            b.append(".").append(output);
        return b;
    }
If the arguments of this contains a single argument which is an identifier, it is returned.
Otherwise null is returned.
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/Reference.java