Chessbase has an interesting article on Babson Problems. A perfect Babson hasn’t been invented yet, but I that would be a far better challenge for Google’s AI rather than the more mundane task of just trying to win games.
Apparently there are 20 Babson problem that have been invented, but all of them either have one or more duels in the main 4 lines, or in one or more of the 4 sidelines.
In chess compositions, what is a sideline? It’s not listed in the wikipedia glossery of Chess Problems.