Two things between a converted EPUB and its chapter titles.
The heading level taken as the chapter map was the shallowest present. Pandoc
writes the book's title as the only `#` and every real chapter as a `##`, so
the shallowest level had one heading in it and the whole book came back as a
single chapter. The dividing level is now the shallowest one used more than
once — a level used once is a title.
And an empty bracketed span survived the heading cleaner, because the pattern
wanted at least one character between the brackets. Pandoc writes an anchor
as exactly that: `## []{#link2H_INTR} Introduction`.
Gibbon's first volume, converted from the Project Gutenberg EPUB: 2 distinct
chapter titles before, 61 after, and none of them carrying an element id.
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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