Italic delimiter normalisation policy. Defaults to Preserve:
structural emit preserves each run’s source delimiter. Fixed
variants are consumed only by the canonicalisation post-pass.
Emission position for collected items (link reference definitions,
footnote definitions). End (the default) gathers items and sorts
them at the end of the document; Preserve keeps source order.
Strong-emphasis delimiter normalisation policy. Defaults to
Preserve. Independent of ItalicStyle so an author can
canonicalise one without forcing the other.
Thematic-break canonicalisation policy. Defaults to Preserve:
structural emit echoes the source --- / *** / ___ line
verbatim. The fixed variants exist for the future canonicalisation
pass.
If source and formatted are not semantically equivalent,
return a short human-readable description of the first divergent
event pair. Returns None if the streams agree.