can you talk a bit more about the use cases here?  I am concerned that this is not easy to reason about since it happens on the mappers. On the reducers, it makes more sense, as you have all the values for a particular key.

I am not inclined to add this for two more reasons: 1) you can clearly easily do this in user code (or with an enrichment), 2) unfold is not such a common pattern (at least to me).

Thoughts?