What's the idea with the $papers stack?

[David Kastrup]

The only place where it is "pushed" is when creating a _new_ book, and
then it is cleared out immediately before it.  What kind of pushing is
that supposed to be?  What does that even mean?  A paper block in a book
definition manipulated the top book entry, except when looking at a book
identifier.  So the following crashes:

xxx = \book {}
\book { \xxx \paper { } }

Does anybody have a clue what the $papers stack is supposed to be for?
When accessing paper variables, LilyPond looks through that "stack".
That's where things like book-specific filenames will be kept.

But other than that?

Is this supposed to mesh with bookparts in some manner?  Those never
even touch the paper "stack".  And books cannot be nested.

-- 
David Kastrup