Yesterday, we (@mrocklin, @clarkfitzg and myself) discussed writing some sort of user facing API for atop for xray that makes use of xray's metadata to automatically generate complex atop calls by inspecting the result of applying the function to a single block.

To make this work generically as a function that transforms xray.Dataset objects, which can be containers of multiple dask arrays, we need a version of atop where the applied function func can have multiple output arguments.