Allow creation of meta-tags, which reference other tags via set operations.   For example, a "#blue" meta tag may include articles tagged with any of "#navy", "#sky", and "#royal_blue" (union). A "#greek_gods" meta tag may cover articles tagged with both "#greek" and "#god" (intersection), while "#flying_birds" may include all "#birds" that aren't "#flightless" (difference).   This would be a quality of life improvement, as the end-result can manually be duplicated by users. Manually adding these tags however both duplicates a lot of data, and is easily error prone from typos. An addition benefit of this is it allows simple and expressive insight into a world's data.