objects, to structure object_refs type requirements as an empty
blacklist, instead of a whitelist. I think it was originally
necessary due to the older implementation of ReferenceProperty
which was in place at the time. With the implementation change
to invert whitelists with generics to blacklists when
allow_custom=True, a "full" whitelist is internally converted to
an empty blacklist anyway, so it winds up being the same thing.
But I think the full whitelist looks better in the code, so I
prefer that to the empty blacklist.
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