Problems with using \bookpart

[David Sumbler]

I am still experimenting with constructing a file structure which makes
sense to me for more complex works.  I have come quite a long way with
it and discovered some interesting things.

But \bookpart seems to cause problems.

The following works:

File: test.ly
%%%%%
\version "2.19.48"

\language "english"

printScore  = ##t
printMvtOne = ##t
printMvtTwo = ##t

$(if
  (and printScore
   (or printMvtOne printMvtTwo))
  #{ \include "testprint.ily" #} )
%%%%%

File: testprint.ily
%%%%%
\version "2.19.48"

\book {
  \score { c }
}
%%%%%

Modify the second file thus:

File: testprint.ily
%%%%%
\version "2.19.48"

\book {
  \bookpart {
    \score { c }
  }
}
%%%%%

This no longer works, but gives the following error messages:

Parsing.../usr/local/lilypond/usr/share/lilypond/current/scm/lily-library.scm:243:5:
 In procedure ly:book-process in expression (process-procedure book paper ...):
/usr/local/lilypond/usr/share/lilypond/current/scm/lily-library.scm:243:5: 
Wrong type (expecting real number): #<undefined>

Once again I am finding that the use of \bookpart is giving problems.  I don't 
know if this is once again somehow connected with the fact that bookparts have 
no variable scope, but it's a damn nuisance anyway!

Any suggestions?

David