(x cby y) is essentially y shifted one step to the right (ignoring the initial value that comes from x). Hence, all values including the boxes move one step to the right. Typically, that moves a box to a place where before there was no box and a place where there was no box may become a real value. Hence, the clocks of y and (x cby y) are usually different ones. Note that the clock is a boolean stream that is 0 if the value is a box, and 1 otherwise.