This function adds a synchronous precedence between the two instances of IlcIntervalVar, x1 and x2. The precedence states that whenever both interval variables x1 and x2 are present, the distance start(x2)-start(x1) between the start of interval x1 and the start of interval x2 is the integer argument z. This function is intended for making binary decisions in regards to the synchronization of the start points of the two intervals.