Python Challenge problem #2

2.rkt

#lang at-exp racket
(define str
@string-append||{
;; giant wall of characters goes here
}||)

(define (frequencies l)
  (for/fold ([h (hash)]) ([i l]) (hash-update h i add1 0)))
(define h (frequencies str))
(apply string
       (for/list ([i str] #:when (= (hash-ref h i) 1)) i))