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Werner Keym
Die Schwalbe, 1979

[4nb1k/p1pppKpB/1p3pp1/88888]

2+11. Last move?

 

Analysis: bPg6 comes from h7. The only way the wK could have reached the f7 square is through a6-b7-c8-d8-e8. So we can't retract b6-b7 before we extract the wK.

Thus Black has no last move. Only the wBh7 can retract a move. If we take back -1. Bg8-h7, we have to go on with -1 ... h7xg6 and the 8th rank is now frozen without any hope of ever thawing.

In fact, extracting the wK requires that the bNe8 gives way (via d6) so that we must first move the wK from f7, i.e. to g8. Thus the bK has to give way, and this is only possible through the h column. But the bK cannot go to h7 because of the wB. Therefore we must first remove this wB, which must be a promoted B !

Solution: We take back -1. Bg8xR/Qh7 R/Qh?-h7 (the R or Q is necessary to avoid Black immediate retro-stalemate) -2. h7xg8=B ... -3. h6-h7 Kh7-h8 and everything unlocks.

Now it is possible to tell what has been captured. Only a Rook can be resurrected on g8 (a Queen or Bishop would give an impossible check, a Knight could not escape). Because the bQR never escaped from the NW cage, and because no promotion by Black could have occurred, Necessarily a bQ has been captured on h7.

Last move was -1. Bg8xQh7. An economical and very rich Type A problem.