Module undo 

Compensating event generation for bn undo.

Events are immutable and append-only. undo does not delete or modify events — it emits compensating events that reverse the observable effect:

Original event	Compensating event
item.create	item.delete
item.update	item.update with prior field value
item.move	item.move back to prior state
item.assign(assign)	item.assign(unassign)
item.assign(unassign)	item.assign(assign)
item.link	item.unlink
item.unlink	item.link
item.delete	item.create (reconstruct from history)

Events that cannot be undone (grow-only by design):

• item.comment — G-Set: comments are permanent
• item.compact — compaction is not reversible without original events
• item.snapshot — same as compact
• item.redact — intentionally permanent

Enums

UndoError

Reason a compensating event cannot be generated.

Functions

compensating_event

Generate a compensating event that reverses the effect of original.