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An alternative could be to remove the constraint and allow to drop grouping variables. This makes more sense than it seems: imagine you are doing resampling and you are only interested in the variation of an estimator, not exactly which group it comes from. But there are implications to this: I don't think it's natural to preserve the grouping when the grouping variables are gone. Hence it would become an implicit ungroup. plyrmr goes with implicit selection, but I am not sure it's the last word.