@Override
    public StringBuilder toString(StringBuilder string, SerializationContext context, Deque<String> path, CompositeNode parent) {
        string.append("!");
        return value.toString(string, context, path, parent);
    }
A node which flips the logical value produced from the nested expression.

@author lesters
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/NotNode.java