[[!comment format=mdwn
 username="joey"
 subject="""comment 2"""
 date="2017-05-09T17:27:35Z"
 content="""
I think that the core primitive here may not be the bidirectional
relationship, but instead setting a metadata field to a key.

If the metadata is known to be a key rather than some other type
of value, then key-specific things can be done with it.

So perhaps the real core thing is to be able to define what the type of a
metadata field is. That could be done using $field-type to define the type
of $field, but it seems it would be better to define it somewhat
globally so that fields with the same name can't have different types.

(The datalad project does something similar for tarballs, using a custom
special remote that retrieves the tarball as an annexed object and then
explodes it to get the requested member object.)
"""]]