Metadata tags sometimes need each other

[GoogleCodeExporter]

Some IPTC tags may only be set when another one is set. This is for example
the case with ReferenceService, ReferenceDate and ReferenceNumber (2:45,
2:47, 2:50). The latter two may only be set if the first one is also set,
and need to follow the first one immediately.

Should we care about this? If yes, how should the user specify this? (The
tags are repeatable, so it's not simply possible to let a writer method
sort them.)

Original issue reported on code.google.com by [email protected] on 20 Mar 2007 at 7:50