What does this smiley :~: stand for?

On 13.01.2016 06:58, David Feuer wrote:
> These are not likely to be *useful*, but they fall easily into the
> general bucket of "exactly one sensible instance exists".
>
> instance Semigroup (a :~: b) where
>    r <> _ = r
>
> instance a ~ b => Monoid (a :~: b) where
>    mempty = Refl
>    mappend r _ = r
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Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel@gu.se
http://www2.tcs.ifi.lmu.de/~abel/
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