This issue is creating an associated MDL for an orphan TODO in tag/lib.php, which I came across while working on [MDL-30992]...

"TODO: turn [our Tag API] into a full-fledged categorization system. This could start by modifying (removing, probably) the 'tag type' to use another table describing the relationship between tags (parents, sibling, etc.), which could then be merged with the 'course categorization' system..."