occurrencesOf's "did not advance" throw is correct and permanent once a bad
purchase is written, so a caller planning to record a confirmation on a date
other than its slot needs to ask, up front, whether that pairing survives the
walk. Extract the rebase-from-a-confirmation rule (previously inlined once)
into `rebasedFrom`, reuse it from the walk itself, and build `wouldAdvancePast`
on top of it and `stepFrom` rather than re-deriving the rule a third time.
No behavioural change to occurrencesOf itself.
Lucas Winther
17f9a44b65
