[Haskell-cafe] Symmetric Difference for Data.Set and Data.IntSet?

[Zemyla]

Could this be implemented as a primitive? I know it can be made from union
and difference, or doing toList on both of them, taking the symmetric
difference, and using fromDistinctAscList on the result, but it'd probably
be much faster if it could be done natively, like can be done with
Data.Map.mergeWithKey.

Alternatively, could we get a merge interface for Set and IntSet roughly
matching Map and IntMap?
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