> use:
>
> ```
> % movement1.ly
> variable = \relative c'' { ... }
> bookpartI = \bookpart { \score { \variable } }
>
> % movement2.ly
> variableII = \relative c'' { ... }
> bookpartII = \bookpart { \score { \variableII } }
>
> % main.ly
> \include "movement1.ly"
> \include "movement2.ly"
> \book {
> \bookpart { \bookpartI }
> \bookpart { \bookpartII }
> }
> ```https://gitlab.com/lilypond/lilypond/-/merge_requests/1868 Werner
