#5 part number of multi volume entries

[Joachim Gabler]

It can happen, that the part number of multi volume
entries is not stored:
- create an entry as part/volume with part number
- store
- delete part number and change entry type from volume
to multivolume entry
- store
- the part number was not stored!

How is determined, whether to store the part number or not?
Better to store it always?