What should the relation between \book / \bookpart be?

I am currently working on books, and with regard to what books and
bookparts contain, there is almost no structural difference (a bookpart
may even contain a paper block using, for example, different page break
algorithms to the rest of the book).

Right now, most of the functionality is determined ad-hoc which is why
you cannot really write something like

bookA = \book { ... }

\bookA

but have to write

\bookA = \book { ... }

\book { \bookA }

instead.  Similar with scheme functions returning a book: they cannot be
used at top level.

So there is a case for not distinguishing them at all in structure and
always require an enclosing \book / \bookpart block for any use outside
of assignments.  Which is more or less what we have now.

That saves us from having to distinguish between

bookA = \book { ... }
bookB = \bookpart { ... }

\book { \bookA ... }

which would mean to create a book as an amended copy of \bookA and

\book { \bookB }

which would mean to create a book containing \bookB as a bookpart.

I am not sure whether our current implementation is perfectly agnostic
in that regard.

So there is some incentive to be able to use either as either.  However,
there is one complication: a book _can_ contain bookparts (but does not
need to).  A bookpart can't.

If one would use a book already having bookparts as a bookpart, it would
need to shed its own bookparts.

The other possibility is to nail down the distinction book/bookpart in
the type/structure.  Either as a default when not explicitly recast, or
hardwired.

Problem either way is that the ability not to specify one's wanted
manner of use makes something like

\book { \bookpartA ...

ambiguous: are we recasting \bookpartA into a book type and amending the
resulting book, or are we creating a book from scratch containing
\bookpartA as its first bookpart?

For rearranging an existing source into something that is only a part of
a larger document, the current relative fluidity between book and
bookpart may be convenient.

On the other hand, expectations are likely that whether something is a
\book/bookpart is more ingrained.

Where to go?

-- 
David Kastrup