x3   x2   x1   x0
                   y3   y2   y1   y0
  __________________________________ x
                 y0x3 y0x2 y0x1 y0x0
            y1x3 y1x2 y1x1 y1x0
       y2x3 y2x2 y2x1 y2x0
  y3x3 y3x2 y3x1 y3x0
  __________________________________ +
z7  z6   z5   z4   z3   z2   z1   z0

z0 = y0x0
z1 = (y0x1 xor y1x0)
z2 = (y0x2 xor y1x1 xor y2x0 xor (y0x1 and y1x0))
z3 = (y0x3 xor y1x2 xor y2x1 xor y3x0 xor
        ((y0x2 and y1x1) or (y0x2 and y2x0) or (y1x1 and y2x0) or (y1x1 and (y0x1 and y1x0)) or (y2x0 and (y0x1 and y1x0)) or (y0x2 and (y0x1 and y1x0)))
...