lhs test
========

``unsplit`` is an arrow that takes a pair of values and combines
them to return a single value:

> unsplit :: (Arrow a) => (b -> c -> d) -> a (b, c) d
> unsplit = arr . uncurry       
>           -- arr (\op (x,y) -> x `op` y) 

``(***)`` combines two arrows into a new arrow by running the two
arrows on a pair of values (one arrow on the first item of the pair
and one arrow on the second item of the pair).

::

    f *** g = first f >>> second g

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