[VEP-003] datalink/core#sibling: Ready for TCG

[Markus Demleitner]

Dear DAL,

In December, we discussed about VEP-003
(https://volute.g-vo.org/svn/trunk/projects/semantics/veps/VEP-003.txt),
which would add a term #sibling to the datalink vocabulary.

Discussion sort of ended with
http://mail.ivoa.net/pipermail/dal/2020-January/008257.html -- and
now I'm not sure whether there are still doubts if #sibling should go
ahead.  Are there?

There was also
http://mail.ivoa.net/pipermail/dal/2020-January/008258.html, in which
Pat suggests #contains and #followup in roughly the same context.  Do
people see these as entangled with #sibling or can we go ahead with
it before thinking more carefully about the other two?

Me, I'd say #contains and #followup may make sense but just don't
match the showcase example for #sibling, the RP/BP spectra and epoch
photometry[1] in Gaia.  These are simply parallel products to the
gaia_source row (which, incidentally, I'd have no qualms to call a
"dataset", too).  They very certainly are not contained in the
gaia_source row, and they're not followups: All these items are
derived from the same observations, although perhaps in different
ways.

         -- Markus

[1] You might call the epoch photometry a #progenitor (though I don't
think that's true in the strict provenance sense) -- but let's
disregard that here.