[jats-list] @seq not allowed on <index> in BITS 2.1?

["denis.maier@xxxxxxxx" <jats-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Hi,

I've just realized that `@seq` is not allowed on the `index` element. Is that
on purpose?

Background: I currently keep the chapters and indexes of a book in distinct
files using the `book-part-wrapper` element, but I'd like to indicate the
order in which they appear in the complete book. With regular book-parts I can
just use `@seq` on `book-part`, but that doesn't seem to be allowed on
`index`. Why is that? Any workarounds? Something else I might be missing?

Best,
Denis