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# An original retro problem #12

• #1

I haven't made one in a while, so here you go.

Which of the following is true:

A) White moved last.

B) Black moved last.

C) Either side could have moved last.

D) The position is illegal.

• #2

either side could have moved last.

• #3

Hmm. It seems that I've created a more complicated puzzle this time.

knightofcrashtest, guessing was not my original intention  when making a claim, you must offer analyse and/or proof game.

• #4

I'm going with C), because this looks like a cook:

• #5

You're absolutely right, it is a cook!

I thought I was being smart with the dark-square bishop, but I missed this. The more complicated the position is, the higher the chance for cooks, unfortunately...

Of course, the answer is C) in this case. However, here's the altered position, with the same question as in post #1. You can do it!

• #6

Black's missing 2P and white made 2 pawn captures.

White's missing RBP and black captured one of them on h6. This leaves 2 captures for bPa7. It must have promoted via b3/a2. That's the only way to sacrifice it to a wP.

The other missing bP is Pd7, which promoted also, and did not capture anything. White can't retract the e6 pawn to d5 until black's pawn returns all the way to d6 or d7. He can't retract a2-a3 until a black pawn is unpromoted from a1 and exits at b3.

-1...Bf7? leaves white with no move to retract. Therefore, white moved last [A)].

• #7

Good analysis and that is the correct answer.

Well done on solving!

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