Here is a transcript of the start of play. The cat's moves are between asterisks ***; the user's moves are signposted with -->.
Miaow, my name is Shadow. The game is to stack 20 cans of cat food. You can choose 1 or 2 or 3 cans at each turn. Me too! First to 20 wins.
Do you want to play first? type y for yes, n for no followed by return.
So, do you want to play first (y/n and press return)? n
*** My move is 2 cans. Total number of cans = 2 ***
enter your number 1, 2 or 3 |: 2
--> You entered 2 cans. Total number of cans = 4
*** My move is 1 cans. Total number of cans = 5 ***
enter your number - 1, 2 or 3: |: ...
And here is a video of my guinea pig, Clare, playing the cat. What was it that Kipling said about winning and losing? (You may have to double-click on video to get full screen).
The game was an exercise from 'The Art of Prolog' and it took me a while to develop the game-play to program. Here is the Prolog program by the way: you may use it to infer the winning strategy.