expr boolean 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.
AMG: To convert a value to its canonical boolean representation (i.e. 0 or 1), you can use [expr {!!$val}].
LV Out of curiousity, why are two ! needed? Doesn't one ! always result in either a 0 or a 1 being returned?