Loose ladder

[michito744]

From #956

The latest version of the b28 network is extremely good and has accurately and quickly resolved many of the aspects we have addressed in past Issues.

Therefore, paradoxically, I suspect that any phase that cannot be solved properly by this network is an algorithmic flaw.

Example of the failure to solve the loose ladder properly: KataGo finally arrived at the right solution after paying a very high cost near the last phase.

This is similar to the case where the old version of the network could not solve the ladder, and may be a weakness of graph search in that it is more complex than the ladder and has a very low probability of being correct.