This is a shortcut for "!foo". Using this method one doesn't have to
worry about how the "!" operator binds. For example, this:
!foo.or(bar)
would be parsed/evaluated as this:
!(foo.or(bar))
when instead we want it to be this:
(!foo).or(bar)
Using explicit parenthesis leads to ugly code, so now we can do this
instead:
foo.not.or(bar)
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