also, i can't understand why, entrance presignals work like combo presignals

some times can happen that a train enter the layout when one train is leaving the shunting yard so all is blocked, i tried to solve this by placing combo presignals one after another...
i think that a best solution to make shunting yards to work well is to use programmable signals, or creating a new signal which works like combo but it become red only if the 2-way entrance presignal is red and don't cares about exit and combo presignals