Allow tags to be tagged? (Tags are a DAG, not a hierarchy!)

One Zotero item can be concerned with multiple topics. Presently, we can create one tag per topic and assign multiple tags to an item. Each tag of an item then represents a "is concerned with" relationship.

Many people have realized that topics can have subtopics, e.g. "Algebra" is a subtopic of "Mathematics". One can argue that every item that is tagged with "Algebra" should also be tagged with "Mathematics". There are many discussions on the Zotero forums where people request "hierarchical tags" similar to Zotero collections, where each tag can have a unique parent tag.

I would like to note that each topic can be a subtopic of MULTIPLE parent topics, e.g. "Physical chemistry" is a subtopic of both "Physics" and "Chemistry". Therefore, instead of a "tag hierarchy", I would like to suggest a system where tags can be tagged with other tags. Each tag A of a tag B then represents a "B is subtopic of A" relationship.

Technically, all tags in a library then form a directed acyclic graph (DAG): If A is a subtopic of B and vice versa, A and B are the same topic and should be represented by a single tag. The same holds if A is a subtopic of B, B is a subtopic of C, and C is a subtopic of A; etc. Thus "closing a tag cycle" should not be allowed (maybe).

From a UI perspective this could be implemented by an additional section below the tag selector that pops up whenever a single tag is selected. That section would allow the user to tag the selected tag with other tags.

Once a tag DAG is set up, the benefit would be that to completely classify a paper, you would only have to tag it with the most specific relevant tags, all parent topic tags could be added or displayed automatically. And when you filter your library for "Mathematics" items, it can never happen that you accidentally filter out an "Algebra" item that you forgot to also tag as "Mathematics".
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