The state of one tree: its identity-bearing elements plus its lesson files. retired carries the tree's declared retirements (each set manifest's metadata.retired_ids, engine#131); absent means "none declared" and keeps pre-existing callers valid.
retired
metadata.retired_ids
Optional
The tree's declared type migrations (each set manifest's metadata.type_migrations, engine#254); absent means none. Only the head's list is read.
metadata.type_migrations
The state of one tree: its identity-bearing elements plus its lesson files.
retiredcarries the tree's declared retirements (each set manifest'smetadata.retired_ids, engine#131); absent means "none declared" and keeps pre-existing callers valid.