include does not find ly's

[Arno Rog]

L.S.

I my everlasting journey to engrave a complete cantata I've stumbled upon the
problem of nested includes. The problem has been mentioned before,
I was unable to find a solution.

I have the following directory-structure (Windows):

 .\
 .\PART1
 .\PART1\MUSIC
 .\PART1\MUSIC\violin.ly

The file score-part1.ly in directory .\PART1 contains
 \include "./MUSIC/violin.ly"
This works just fine
 
The file partitur.ly in directory .\ contains
 \include "./PART1/score-part1.ly"
This does not work, it is unable to find violin.ly

Is there a way to achieve this without using hardcoded path's?

Thanks in advance,

Arno Rog