As currently defined, `shift x minBound` calls `shiftR` with a negative displacement.

This would avoid that:
```
shift x n
| n >= 0 = shiftL x n
| n == minBound = shiftR (shiftR x maxBound) 1
| otherwise = shiftR x (-n)
```

P.S. Philosophically, `rotate` on a negative `Integer` should do sign extension in the least significant bits (though by the description of `rotate` it should not...).

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