This is why, when people start asking for categories or hierarchies, you implement it as tags without telling them, if it's at all possible to do so.
Then when they decide they actually need tags, you either get to be a hero, or to goof off for a couple weeks, depending on your situation :-)
Nesting works the same way. "OK we said we needed N levels but actually we need arbitrary levels of nesting, and also we don't realize it but what we're asking for can introduce cycles to the structure". Every. Damn. Time.
It's a tyranny of hierarchical data-structures and ontologies. Always breaks down. Use relational models, graphs, tags, sets, uuids/global identifiers etc. Trees are an optimization strategy/snapshot and almost never a model for the real world.
Then when they decide they actually need tags, you either get to be a hero, or to goof off for a couple weeks, depending on your situation :-)
Nesting works the same way. "OK we said we needed N levels but actually we need arbitrary levels of nesting, and also we don't realize it but what we're asking for can introduce cycles to the structure". Every. Damn. Time.