;'Egyptological' multiplier. XY_Lo and XY_Hi hold result of X*Y.
(procedure fz_mul_egyptian
           ((x      (fz :in))
            (y      (fz :in))
            (xy_lo  (fz :out))
            (xy_hi  (fz :out)))
           ((l  (indices :constant) (length x))
            ;Register holding running product
            (xy (fz 1 (+ (length x) (length y))))
            ;X-Slide
            (xs (fz 1 (+ (length x) (length y)))))
  ;Product register begins empty
  (fz_clear xy)

  ;X-Slide initially equals X:
  (set (slice xs 1                (length x)) x)
  (set (slice xs (1+ (length x))  (last xs))  ((:others . 0)))

  ;For each word of Y:
  (for (i (range y))
        ;Current word of Y
       ((w    (word)              (nth i y))
        ;Current cut of XY and XS. Stay ahead by a word to handle carry.
        (cut  (indices :constant) (+ l i))
        (xyc  (fz :renames (slice xy 1 cut)))
        (xsc  (fz :renames (slice xs 1 cut))))
    (for (b (nrange 1 bitness))
      ;If current Y bit is 1, X-Slide Cut is added into XY Cut
      (fz_add_gated :x    xyc
                    :y    xsc
                    :sum  xyc
                    :gate (and w 1))

      ;Crank the next bit of Y into the bottom position of W
      (set w (shift_right w 1))

      ;X-Slide := X-Slide * 2
      (fz_shiftleft xsc xsc 1)))

  ;Write out the Product's lower and upper FZs:
  (set xy_lo (slice xy 1                    (length xy_lo)))
  (set xy_hi (slice xy (1+ (length xy_lo))  (last xy))))