Few answers under this question have inspired me with the interesting idea of using Zotero "folders" (called collections) as hierarchical tags. Zotero does allow to create folder (=collection) tree in the left pane, and one reference can be put in more such collections. But, is Zotero able to search references present in two or more collections? That would be super cool, because then we could use those collections as tags!
Actually, the tool is there))
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Steps (look at the screenshot).
- Open advanced search
- Choose "Collection" as a search parameter
- Choose collections to search in
- Add more that one collection