; this is assuming that the input is case-insensitive
; despite a unique word count implying words and not 'words with following or 
; leading punctuation', this will terminate only on space or newline

(define (trie-new)
  (make-vector 256 0))

(define (trie-insert tr s)
  (let ((tr   (trie-new))
        (l    (string-length s)))
    (let loop ((i  0)
               (t  tr))
      (if (= i l)
          (begin
            (vector-set! t 0 (+ 1 (vector-ref t 0)))
            tr)
          (let ((c  (char->integer (char-downcase (string-ref s i)))))
            (cond ((or (= 32 c) (= 10 c))    ; 32 = #\space   10 = #\newline
                    (vector-set! t 0 (+ 1 (vector-ref t 0)))
                    (loop (+ 1 i) tr))
                  ((vector? (vector-ref t c))
                    (loop (+ 1 i) (vector-ref t c)))
                  (else
                    (let ((tn   (trie-new)))
                      (vector-set! tn 0 (vector-ref t c))
                      (vector-set! t c tn)
                      (loop (+ 1 i) tn)))))))))