Write multi-part file

Hello,

I have read all the documentation that comes with OpenEXR and searched 
the internet for hours but I just can't figure out

how to write multi-part exr files.

It is stated that this and deep images are new to v2 of the standard but 
the documentation only covers those deep images.
So could someone please, please, please give me a hint on how to 
generate such files?
Best would be a compilable cpp file as one can find for the other "exr 
use cases".
I imagine is has just another loop around the
for( x=0...)
   for( y=0...)

like
for( part=0...)
   addNewPart()
   ...

Hoping for some help,
Chris