[plt-scheme] optional arguments in function contracts

HI,

I'm trying to do a contract with an optional argument, but I'm stuck
(and can't find an example in the doc's, tests or 'plt-full' code)
my provide/contract is;
;; write-email accepts an email and optionally an output-port (2 or one args)
(provide/contract
 (write-email (email? (or/c none/c output-port?) . -> . any))
 )


and my function is
;; write-email : message [output-port] -> void
(define write-email
  (lambda (message (out-port (current-output-port)))
    (printf "~V:~V~N" message out-port)
    )
  )

but calling it I get the error; procedure write-email: expects 2
arguments, given 1:

Forgive me if this is a dumb question but I feel I'm missing something
fundamental about how I'm specifying the contract?


Cheers,

Stephen

--
Stephen De Gabrielle
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