A two-mover which presents a strikingly original combination of twinning and theme.
In a) the check at a1 must be prevented, so 1.O-O threat 2.Bxe5. In b) we must consider the previous play. White’s only missing piece is a knight, which was captured on b6 before the bishop reached b8. The black king could not have played last from f4, as White’s previous move could not have been e2-e3+ (the bishop at g8 could not have got out) or a capture from d2 or f2, as that would mean that White had earlier played six pawn captures but only five pieces were available for capture. If the last move was Pe6 to e5 White could not have played the previous check. The last four moves (going backwards) must have been e7-e5, Rd6-h6+, Kh4-g3 or Kh2-g3, Rh1+, hence castling is illegal and the solution is 1.dxe5 e.p.+ Kxg2 2.Qd5. In c) the black knight can move to c4 or c6, invalidating the castling solution, so 1.f8Q threat 2.Qf2.The combination of the Valladao theme in the three key moves with Forsberg type twinning is highly original.
Jacob Hoover: Since castling, pawn promotion, and en passant are all here, we have the Valladao theme. This problem was quite a difficult solve.
Juha Neitola: b) offered a tricky challenge. The black e5 pawn seems likely to be captured, but one has to figure out the preceding four half-moves to see the unlikely capturing move that allows an equally unexpected queen mate, while castling is proved illegal.