containers: intersections for Set, along with Semigroup newtype

[Sven Panne]

Am So., 6. Dez. 2020 um 07:20 Uhr schrieb Reed Mullanix <
reedmullanix at gmail.com>:

> [...]
>   intersections :: Ord a => NonEmpty (Set a) -> Set a
>   intersections (s :| ss) = Foldable.foldl' intersection s ss
> [...]
>

Why NonEmpty? I would expect "intersections [] = Set.empty", because the
result contains all the elements which are in all sets, i.e. none. That's
at least my intuition, is there some law which this would violate?
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