@Override
    public StringBuilder toString(StringBuilder string, SerializationContext context, Deque<String> path, CompositeNode parent) {
        testValue.toString(string, context, path, this);
        string.append(" in [");
        for (int i = 0, len = setValues.size(); i < len; ++i) {
            setValues.get(i).toString(string, context, path, this);
            if (i < len - 1) {
                string.append(", ");
            }
        }
        string.append("]");
        return string;
    }
Returns an immutable list of the values of the set
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/SetMembershipNode.java