; doubled letters
 
(define (double? str)
  (let loop ((cs (string->list str)))
    (cond ((or (null? cs) (null? (cdr cs))) #f)
          ((char=? (car cs) (cadr cs)) #t)
          (else (loop (cdr cs))))))
 
(define (complement f) (lambda xs (not (apply f xs))))
 
(define (remove-doubled words) (filter (complement double?) words))
 
(display
  (map string->symbol
    (remove-doubled
      (map symbol->string
        '(Now is the time for all good men to come to the aid of their country)))))

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