The 'Part of a set' frame is a numeric string that describes which
part of a set the audio came from. This frame is used if the source
described in the "TALB" frame is divided into several mediums, e.g. a
double CD. The value MAY be extended with a "/" character and a
numeric string containing the total number of parts in the set. E.g.
"1/2".
@author Eric Farng
@version $Revision: 1.4 $