The chapter a passage carries was resolved at the offset of the page it came
from, not the offset of the passage. A document with no page markers is a
single page beginning at zero, so every chunk in such a book answered with
whichever heading came first in the file — and took the role classification
with it, since that reads the chapter title.
Measured over a corpus of 5,386 books: all 1,232 unpaginated books with
twenty or more chunks carried exactly one chapter title from cover to cover,
as did 73% of the paginated ones. A bibliography chapter was therefore
invisible to anything selecting on the chapter title, which is the first of
the three ways the citation stage finds one.
The packer now hands back each passage with the offset it began at, and the
page's own offset is the true start of its text rather than the position of
the marker before it.
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