public Topic AddSubTopic(string topic)
        {
            string topicName = topic.ToLower();
            Topic subTopic = GetSubTopic(topicName);

            if(subTopic == null)
            {
                subTopic = new Topic(topicName, this);
                this.SubTopics.Add(topicName, subTopic);
            }

            return subTopic;
        }
        public Topic(string topicName, Topic parent)
        {
            this.TopicName = topicName.ToLower();
            this.parent = parent;

            if(parent == null)
            {
                this.fullName = "/";
            }
            else
            {
                this.fullName = parent.fullName + topicName + "/";
            }
        }