Retract and helpmate

Black and White retract one move each (Black retracts first), and helpmate in 2.5 moves (starting from White, cooperate to mate Black after White's 3rd move, not counting the retraction).

This is overly cooked; I'm more looking toward whether the main idea is the same as mine.

One solution is that Black retracts Kh2h1 and White retracts Nh4xf3.

The white knight could also have come from d4, g5 or d2.

Another class of solution has Black uncapturing a bishop.

In the forward solution, White plays 1. Bg2 2. Kf2 3. Nf3# while Black just moves his knight/bishop away from f3.

If you want to get really nitpicky, black MUST retract -1...Kh2xBh1 as the last move. It can only be an uncaptured bishop due to DR (dead reckoning).

Other than that minor quibble, looks like caveatcanis got 2 sound ideas.

I didn't notice the second class of solutions, yeah. I was only thinking of the first one, where king and knight mates king and knight. (Or king and knight vs king and bishop, but then the color of the bishop is wrong.)

DR only applies if stipulated. It would make some existing problems unsound if it applied by default.

Since July 2015, the Codex confirms that DR applies by default to retro problems only. Retractors are retros. There are endgames studies where a pretty stalemate would never be encountered if DR applied. Similarly selfstalemates & some helpstalemates. At the time that this comment was written (Feb 2014) actually all DR did indeed disrupt these problems. That's why the Codex modification was brought in by the endgame study community.

Actually, isn't -1...Kh2xNh1 ok DR-wise? Mate with two N's is possible with cooperation.

And so it is.