[racket] Optional parameters in Typed Racket?

According to Sam according to google

http://lists.racket-lang.org/users/archive/2011-November/049010.html

(: append-bar (case-> (String Positive-Integer -> String)
                      (String -> String)))
(define (append-bar str [how-many 1])
  (apply string-append str (make-list how-many "bar")))

How come pasting in gmail expands the previous email quote?
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