1	import Base
2	
3	# Names the visibility boundary reached before a durability failure.
4	type Phase is Data:
5	  BeforeRename{}
6	  AfterRename{}
7	
8	# Distinguishes a definite failure from an uncertain published result.
9	type Failure is Data:
10	  KnownFailure{}
11	  CommitUnknown{}
12	
13	# Classifies whether a publication failure leaves destination visibility uncertain.
14	def failure(phase: Phase) -> Failure:
15	  match phase:
16	    case BeforeRename{}: KnownFailure{}
17	    case AfterRename{}: CommitUnknown{}