Hi David,

Thanks for responding.
Let me re-phrase the technical question: in some hypothetical

>       instance Foldable Enumerator where ...

the methods (e.g., foldMap) would be overconstrained.
Is there a way to still write something like it?

It seems not, as shown by these examples:

Data.EnumSet cannot implement Foldable because of  Enum k.
http://hackage.haskell.org/package/enummapset/docs/Data-EnumSet.html

Data.IntSet cannot implement Foldable because of   k ~ Int.

- J.
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