[expr] [bool]ean negation operator

returns truth (1) if the argument is a falsehood, and falsity (0) of the argument is true

For bitwise "not" use the [~] operator.

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[AMG]: To convert a value to its canonical boolean representation (i.e. 0 or 1), you can use `[[expr {'''!!'''$val}]]`.

[DGP]: The bool() function is the recommended way these days.
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