[racket] An easy route to define-datatype in BSL?

On Mon, Nov 5, 2012 at 7:22 AM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:

> every top-level define (like for 'f') has a special notation on it that
> says "I'm a function". Since 'a' doesn't have this, then the macro for app
> in BSL can throw an error.
>

What Racket language feature is used to do that?
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