[racket] Holzweg-PLAI. Not doable in DrRacket 6.0.1?

Hi -- thanks for the report. However, you should be using plai-typed, not plai.

You can find plai-typed here:

http://cs.brown.edu/courses/cs173/2012/lang/

If you install and run with plai-typed instead of plai, you will find
that the colon-endowed syntax works just fine (I just tested the very
code you pasted).

Note that the book section you point to also says "plai-typed".

Thanks,
Shriram