Tags to visualize subsequences of topics

It's difficult to identify which topics are missing without visualizing subsequences. For example, if I saw "add within 10" and "add within 100," with no "add within 20" in between, I would know that I'm missing that topic. Thus, I've tagged certain interesting subsequences. So far I've tagged these: arithmetic, measurement, classifying-shapes. Once the topics have been tagged, I can use list_topics_by_tag(tag) to visualize the subsequence.