This function adds a 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 must be greater than or equal to the integer argument z. This function is intended for making binary decisions relative to the start points of two intervals.