HorizontalRule corresponds to <hr> element in the default output
format, HTML. Current HTML standard defines <hr> element as
"paragraph-level thematic break". In typography it is often
represented by extra space or centered asterism ("⁂"), but since
FB2 does not support text centering, empty line (similar to extra space)
is the only solution.
Line breaks, on the other hand, don't generate <empty-line />
anymore. Previously line breaks generated <empty-line /> element
inside paragraph, which is not allowed. So, this commit addresses
issue #2424 ("FB2 produced by pandoc doesn't validate").
FB2 does not have a way to represent line breaks inside paragraphs.
They are replaced with LF character, which is not rendered by
FB2 readers, but at least preserves some information.
Only <p> and <empty-line /> are allowed in titles,
but <p> has the same type as an ordinary paragraphs.
Therefore, there is no need to remove emphasis from titles.
Also, don't intersperse paragraph with empty lines.
<annotation> is not allowed inside <body> according to FictionBook2 XML schema. Besides that, the same information is already placed inside <description>.
Related bug: #2424
According to FB2 XML schema <empty-line /> cannot be placed inside
<p>. Linux FBReader can't display such paragraphs, e.g. any "loose"
lists produced by pandoc prior to this commit. Besides that,
FB2 writer placed <p> inside <p> when writing nested lists,
this commit fixes the bug.
Also this commit removes leading non-breaking space from ordered
lists for consistency with bullet lists.
Definition lists are not affected at all.
Note that as a result of this change, the following,
which formerly produced a header with two lines separated
by a line break, will now produce a header followed by a
paragraph:
# Hi\
there
This may affect some existing documents that relied on
this undocumented and unintended behavior.
This change makes pandoc more consistent with other
Markdown implementations, and with itself (since the two-space
version of a line break doesn't work inside ATX headers, and
neither version works inside Setext headers).
Closes#3730.
