[racket] happy april fools day: arctangent 0.1

>
> BTW, Arc does seem like a pretty good example to use in a tutorial, since
> you can mostly use the normal Racket reader, yet the language looks
> different enough, and the implementation is nontrivial.  Chapter 2 of the
> tutorial uses the example of translating Python? :)

We've got the lexer:

    https://github.com/dyoo/python-tokenizer

and a parser:

    https://github.com/dyoo/autogrammar/blob/master/examples/python-grammar.rkt
    https://github.com/dyoo/autogrammar/blob/master/examples/test-python-grammar.rkt