[poppler] Spec question: Incremental updates and mixing XRef tables and streams

[Fabio D'Urso]

I'm trying to fix https://bugs.freedesktop.org/show_bug.cgi?id=49080.

According to my tests, it seems to me that it's not possible to incrementally-
update a document indexed by a XRef stream by appending a XRef table and 
making /Prev point to the previous XRef stream (which is what poppler 
currently does). In such cases acroread says:
There was an error opening this document. There was a problem reading this
document (14).

OTOH, if I append a XRef stream to a document that already has an XRef table 
(again, pointing it from /Prev), acroread briefly shows a "I'm recovering a 
damaged document" message, but then it opens the file.

My question is whether is it allowed to mix cross-reference tables ans cross-
reference streams in the chain of incremental updates?

Thank you,
Fabio