My new one, the existing formula not deduction, turn to the great god! Account only 20 points, all gave,
I (y1, y2; S)=I (y1; S) + I (y2; S | y1)
I (y1. S)=H (y1) - H (y1) | s this step, I know, but I (y2. S | y1) is derived according to the definition?