For future reference - the da.unique function was not used in this PR, we used da.reduction instead, because the former does not deal very well with missing data. I had a look at modifying da.unique to cope, but a) it was not trivial; b) cf.Data doesn't need (yet) the return_* keywords; and c) the dask folk might change it to be a reduction at some stage (dask/dask#2851). For these three reasons, it was preferable at this time to implement a missing-data-friendly "unique", without the return_* keywords as a standard cf.data.Collapse method.