BigDoggProblem wrote:
The first one is a interesting problem in that it always comes down to one piece not getting back, and that one can be several different pieces.
I didn’t think about retracting pieces back home (apart from Ra8, which goes home once h6-h7 is retracted): I was thinking more about forward moves reaching the position, with wBg8, wPh6, bBc8 and bPs trapping the royalty. But I guess you are right - it is interesting when you consider retracting all pieces back home.
In fact, all those 4 pieces which you had mentioned in your illegality proof can return home, but only if the last move is Qe1xQf1+. This prevents a different type of piece (Bc8) from returning home.
#8676
Black is apparently missing 2P and 1 N. White made 3 captures.
White is missing LSB, which was captured on f1, and N, which must have been captured on the c-file. Black made no other captures, meaning a and f pawns promoted straight down.
If only white could retract f2xe3, the position would unlock, but he must wait until black unpromotes on f1 and retracts that P to f3.
The best bet is an N promotion on f1, but it just isn't fast enough. White's g pawn can uncap an N at several places, but white runs out of pawn retractions with black's P on f2 in all tries.
Nice work with both positions! The first position was meant to be slightly harder than the second, but you still provided brilliant and concise analysis for both.
The second position seems to be just another case of white running out of tempo by one move. However, it does feature some rather interesting knight geometry: a N on f1 takes 1 move to g3, 2 moves to g4, 3 moves to g5, and 4 moves to g6. Of course, the sooner white uncaptures the bN, the more pawn tempi available, but also more moves for the bN to retract to f1.
The first one is a interesting problem in that it always comes down to one piece not getting back, and that one can be several different pieces.