(define (fxreverse-bit-field61 v)
    ;; Based on <http://aggregate.org/MAGIC/#Bit Reversal>.
    (assert (= (fixnum-width) 61))
    (let* (;; Swap pairs of bits
           (v (fxior (fxarithmetic-shift-right (fxand v #b101010101010101010101010101010101010101010101010101010101010) 1)
                     (fxarithmetic-shift-left  (fxand v #b010101010101010101010101010101010101010101010101010101010101) 1)))
           ;; Swap 2-bit fields
           (v (fxior (fxarithmetic-shift-right (fxand v #b110011001100110011001100110011001100110011001100110011001100) 2)
                     (fxarithmetic-shift-left  (fxand v #b001100110011001100110011001100110011001100110011001100110011) 2)))
           ;; Swap 4-bit fields
           (tmp1     (fxarithmetic-shift-right (fxand v #b111100000000000000000000000000000000000000000000000000000000) 56))
           (v (fxior (fxarithmetic-shift-right (fxand v #b000011110000111100001111000011110000111100001111000011110000) 4)
                     (fxarithmetic-shift-left  (fxand v #b000000001111000011110000111100001111000011110000111100001111) 4)))
           ;; Swap bytes
           (tmp2     (fxarithmetic-shift-right (fxand v #b000011111111000000000000000000000000000000000000000000000000) 44))
           (v (fxior (fxarithmetic-shift-right (fxand v #b111100000000111111110000000011111111000000001111111100000000) 8)
                     (fxarithmetic-shift-left  (fxand v #b000000000000000000001111111100000000111111110000000011111111) 8)))
           ;; Swap 16-bit fields
           (tmp3     (fxarithmetic-shift-right (fxand v #b000000000000111111111111111100000000000000000000000000000000) 20))
           (v (fxior (fxarithmetic-shift-right (fxand v #b111111111111000000000000000011111111111111110000000000000000) 16)
                     (fxarithmetic-shift-left  (fxand v #b000000000000000000000000000000000000000000001111111111111111) 16))))
      ;; Put together the pieces
      (fxior (fxarithmetic-shift-left v 28)
             tmp1 tmp2 tmp3)))

  (define (fxreverse-bit-field30 v)
    (assert (= (fixnum-width) 30))
    (let* (;; Swap pairs of bits
           (tmp1     (fxarithmetic-shift-right (fxand v #b10000000000000000000000000000) 28))
           (v (fxior (fxarithmetic-shift-right (fxand v #b01010101010101010101010101010) 1)
                     (fxarithmetic-shift-left  (fxand v #b00101010101010101010101010101) 1)))
           ;; Swap 2-bit fields
           (v (fxior (fxarithmetic-shift-right (fxand v #b01100110011001100110011001100) 2)
                     (fxarithmetic-shift-left  (fxand v #b10011001100110011001100110011) 2)))
           ;; Swap 4-bit fields
           (tmp2     (fxarithmetic-shift-right (fxand v #b01111000000000000000000000000) 23))
           (v (fxior (fxarithmetic-shift-right (fxand v #b10000111100001111000011110000) 4)
                     (fxarithmetic-shift-left  (fxand v #b00000000011110000111100001111) 4)))
           ;; Swap bytes
           (tmp3     (fxarithmetic-shift-right (fxand v #b00000111111110000000000000000) 11))
           (v (fxior (fxarithmetic-shift-right (fxand v #b11111000000001111111100000000) 8)
                     (fxarithmetic-shift-left  (fxand v #b00000000000000000000011111111) 8))))
      ;; Put together the pieces
      (fxior (fxarithmetic-shift-left v 13)
             tmp1 tmp2 tmp3)))

  (define (fxreverse-bit-field v start end)
    (assert (< -1 end (fixnum-width)))
    (assert (<= 0 start end))
    (assert (fixnum? v))
    (cond ((= (fixnum-width) 61)
           (fxior (fxarithmetic-shift-right
                   (fxreverse-bit-field61 (fxbit-field v start end))
                   (fx- 60 end))
                  (fxcopy-bit-field v start end 0)))
          ((= (fixnum-width) 30)
           (fxior (fxarithmetic-shift-right
                   (fxreverse-bit-field30 (fxbit-field v start end))
                   (fx- 29 end))
                  (fxcopy-bit-field v start end 0)))
          (else
           (do ((i start (fx+ i 1))
                (ret 0 (if (fxbit-set? v i)
                           (fxior ret (fxarithmetic-shift-left 1 (fx- (fx- end i) 1)))
                           ret)))
               ((fx=? i end)
                (fxior (fxarithmetic-shift-left ret start)
                       (fxcopy-bit-field v start end 0)))))))