[racket] Meaning of "@" notation in TR

Hi all,

I'm puzzled by this cryptic notation in Typed Racket (copied from the docs):

(λ:<http://docs.racket-lang.org/ts-reference/special-forms.html#(form._((lib._typed-racket/base-env/prims..rkt)._~ce~bb~3a))>
 ([x : Number<http://docs.racket-lang.org/ts-reference/type-ref.html#(form._((lib._typed-racket/base-env/base-types..rkt)._.Number))>
]) x)

- : (Number -> Number : ((! False @ 0) | (False @ 0)) (0))



What does the "@ 0" mean?  I have searched all of the TR doc pages, but I
see no explanation.

Thanks,
jmj
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