some working grob properties are unlisted?

[Mark Polesky]

I noticed that the 'baseline-skip property works with the
InstrumentName grob, which confuses me, because in the IR
entry for IntsrumentName, none of the supported interfaces
listed at the bottom provide the 'baseline-skip property.

Should InstrumentName support text-interface, which provides
'baseline-skip?  Or should some properties from
text-interface also be added to system-start-text-interface,
which *is* supported by InstrumentName (and if so, which
ones)?

But more importantly, how does a property work when it's not
provided by any of the grob's interfaces?  Ambiguities like
that bother me.

Mark