For now I found those 2 problems in the implementation of Kingdrops:
1. no piece can move more than once per turn: I have found a solution for
2-moves per turn ("verify not-last-to?") but I'm without solution for my
3-moves per turn
2. how can I restrict the dropping zone to the king's 8 adjacent squares?
3. the king has to get out of check in the fist move... but how? (maybe
this is impossible in zillions?)