[racket] happy april fools day: arctangent 0.1

I'm announcing the following PLaneT language: Arctangent.  As the name
suggests, this is not Arc.

Here's a small example:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
#lang planet dyoo/arctangent

(= foo 13)
(prn foo)
(= foo (+ foo 1))
(prn foo)

(def hypo (a b)
  (= a^2 (* a a))
  (= b^2 (* b b))
  (sqrt (+ a^2 b^2)))

(prn (hypo 3 4))

(= message "hallo")
(prn (message 1))
(= (message 1) #\e)
(prn message)

(def translate (sym)
 (case sym
   apple 'mela
   onion 'cipolla
         'che?))

(prn (translate 'apple))
(prn (translate 'syzygy))

(prn (map [+ _ 10] '(1 2 3)))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Other examples of expressions can be found in the test suite here:

    https://github.com/dyoo/arctangent/blob/master/test-arctangent.rkt


There is no documentation yet.  I intend to use Arctangent as the
motivating example for a introductory macros-and-modules guide.  I
hope I have the time to write it soon, but I ran out of time before
April 1st.