Reverse Chess Engine?

Hi everyone, this is my first post on this forum, so please be a bit patient with me.

To be honest, I found this thread slightly infuriating, which caused me to create an account to comment

I had the same question yesterday and after some googling around, I found the answer. It's called 'Retrograde analyses' (which is quite a fitting name for... analyzing games while stepping backwards).

It even has a wikipedia article: https://en.wikipedia.org/wiki/Retrograde_analysis

...and a Chess programming wiki article, too: https://www.chessprogramming.org/Retrograde_Analysis

tl;dr: People are considering the problem since early 19th c., and there seem to have been some recent advances.

The threads original question could be re-stated as: "Is there is any engine that implements the un-move".

"The un-move generation is similar to move generation, with the difference that it is illegal to start in check, but legal to un-move into check, and illegal to capture, but legal to un-capture by leaving an opponent piece behind."

Also, in snoozymans quiz, answer f (10e50) is correct, not g (10^100). I'll try to explain it visually:

0% ..............................1%............................................................................................................. 100%

------------------------------------------------------------------------------------->

0 1 2 ... 10^50... 10^98........................................................................................................ 10^100

If (10^98) is only 1% of (10^100), and all the other options are smaller than (10^98), then all other options have to fall "between zero and one percent" on the number line, which leaves a 99% large gap to 10^100. So the answer has to be 10^50.

Please correct me if I'm wrong. Also, sorry for ... disturbing this peacefully sleeping thread, I guess.