1+5. Black to move. What did White just play?
White did just move his King to e5, but from which square? Whatever square we try, the King was in a seemingly impossible double check.
As a matter of fact, one such double check is possible. If we assume White did just play n. Kf5-e5, then the double check on f5 could have been given by an en-passant capture n-1 ... f4xg3 e.p.
But then the capture was only legal if White did play n-1. g2-g4. This move answered a check given by the bBh3, again seemingly impossible. The trick is that this must have been a discover check, given by some bNg4 that has been captured by the wK.
Last moves are:
n-2 ... Ng4-(/x)e5 +
n-1. g2-g4 f4xg3 ep +
In retro-parlance, this cute problem is a "Last move (Type B)" problem with 3 precise last single moves. The 4th retro-move is not precise because it could have been any capture Ng4xe5.