i.e. a custom playmode

You could have tags which tell your playmode whether a track
should always have a particular track directly afterwards (when
the two tracks are actually part of a larger work, for instance).

You could have other tags that the playmode uses to decide that it
should never play track B without playing track A first (you might
want also this for different movements of the same work).

Of course, a playmode is free to ignore such tags if it wants.

> Not an easy problem for the programmers to solve, eh?

I personally believe it is an impossible problem. I guarantee that if
complex things like this start getting implemented, users are going
to come with new ones much faster than they can be coded.

Richard.