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Note that this only makes sense if we’re splicing from multiple source Units. Otherwise, it’s enough to just put the upstream indicator on the vertical itself.
An alternative approach that would be more backwards compatible for sequentials and other things that expect to have a specific set of child elements would be to create a new upstreams (plural) attribute that has name → usage key mapping. Or if we want to make it even less possible to write OLX validation, we can make dynamic upstream fields like upstream-intro-to-openedx.
Multiple inheritance is edge-case city though. We should probably just ignore default values altogether rather in that case, instead of trying to combine things.