Feature request: groupby on columns with overlapping column combinations
Currently I am not able to use groupby on columns to produce groupings that contain combinations of same columns. Let's say I have a dataframe with columns A, B , C and D. I would like to create groups that contain [A,B], [A,C] and [B,D]. 

One obvious way to implement this would be to allow as argument a dictionary of lists, where the keys are output labels and the lists are the column labels to be grouped. 

I am aware that currently grouping via dictionary works the opposite way, i.e. the key is the input label and the value is the output label, but at least to me this seems somewhat less useful than the reverse. 