Removed `Eq` from `Expression`
Removed `Eq` from `Expression`

The reason is, as @CarstenKoenig has pointed out on Twitter
(https://twitter.com/CarstenK_Dev/status/919849880172007424),
`Expression` isn't associative if one uses the `Eq` instance to evaluate
equality. It's still associative if evaluated using `reduce`, as described
in the C# code example.

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