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      I don’t see why to disallow // on NonEmpty. Sure it will never return the default case, but that’s fine.

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        There might be no mathematical reason to exclude it, but using (// dfault) instead of head for a NonEmpty would make me stop and question whether I or the author misunderstood what it would do.

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          If they know they have a NonEmpty, sure, but since it’s a polymorphic operator I would expect it to sometimes be used in a polymorphic context.