The ring said which folder a reading was asked of by taking the row's own
name apart and comparing the pieces, which is the same path spelled twice and
only nearly the same. When the two spellings drifted the asked row never
matched, and the fallback that used to carry the mark on the roll's row had
gone, so a reading ran with nothing on the column at all.
The mark is now built with the key the walk gives the row — `folder/sub`,
through `normPath`, the same way a frame id is spelled — and a reading whose
own row is not on the column (a branch folded away, a folder gone from the
disk) falls back to the roll's own row, so a reading is never invisible.
3dtours
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