Yeah, the key insight I think is that if you don&#x27;t put any constraints on some variable then the intent is that the equation is true for any choice of that variable- so since x_i = y_i for any i, then it&#x27;s logically the same as saying x0 = y0, x1 = y1, ...   It was an aha moment when I realized what they were doing there.  Very elegant actually.