Fwd: Boolean equivalent to pre-increment and post-icnrement

[Bob Myers]

In the stupid idea of the day department, for some reason I have felt the
urge more than once in recent months for an operator which would invert the
value of a boolean variable while evaluating to its pre-inversion value.
For example:

```js
if (bool!!) console.log("used to be true");
```

The post-inversion case is less important since I can just write `if (bool
= !bool)`.

Bob
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