In order to mark up general references on the documentary level while using <line>, <rs> should be made a member of model.linePart.
Key questions to be considered:
Should the user be allowed to mix documentary markup with elements of a rather 'interpretative' encoding?
How about the pitfalls of overlapping structures?
Citing Gerrit Brüning from the mailing list:
<line> was invented to allow a rigorously documentary encoding.
It was not invented to draw a fix line between a purely documentary
and a more 'interpretative' encoding. (There are people who even deny
the mere differentiation.)