References ▸ Index entry marks a place with the term the index should
list — under a key of its own where it belongs elsewhere — and the Index
in the same menu lists each term once, sorted case-insensitively, with
every page it was marked on.
Both formats write a place, not a range, so the entry is an inline atom:
ODF text:alphabetical-index-mark plus text:alphabetical-index, Word XE
plus INDEX. LibreOffice reads our file back unchanged. The bibliography,
the other half of that gap, is still open.