Document the ⊢ function/syntax/thing.

[LeifAndersen NA]

In the video paper, you use ⊢, which seems to generate reference errors. It seems like it would be a good idea to document it to get rid of those errors.

[Stephen Chang]

Documented here I believe, http://docs.racket-lang.org/turnstile/The_Turnstile_Reference.html?#%28form._%28%28lib._turnstile%2Fturnstile..rkt%29._define-typed-syntax%29%29