I have a bit of a problem with my cats that has resulted in me needing to give one cat full access to the outside (at least during the day) but not the others. I've been looking at microchip cat flaps on Zooplus, but I'm a bit confused by some of the reviews. One reviewer has said you can restrict which cats get in, but can't restrict which cats get out. I would have thought a microchip flap would only let "approved" chips through it in either direction. Can anyone who has one of these confirm exactly how they work?