Consider allowing multiple values in a single "ref"

[tomdonaldson]

(This topic was mentioned on the Applications mail list.)

While exercising the new TIMESYS element for VOTable 1.4, examples were encountered where multiple ref values would have been helpful. Those examples should be described in this issue to further the discussion.

One proposal for addressing this would be to change the type for ref from IDREF to IDREFS which allows a space-separated sequence of IDREF. We would need to assess the consequences of such a change, remembering that ref is used for many elements, not just TIMESYS. Perhaps that also brings up the question of whether the generic ref should be augmented or replaced by more explicit attributes such as timeref.