Pilots love to complain about unstacking in PBS. Keep in mind regular line bidding is 100% unstacked. They do not leave trips on holidays and weekends in open time. They are all built into the lines. PBS on the other hand can and usually is programmed to allow some stacks to develope over holidays and even weekends. How big or little the stacks run is determined by the negotiated work rules. Very few airlines with PBS have 100% unstacking.