The phrasing "Legal but non-reachable " makes clear that they use some notion of legal that differs from the normal one of reachability. It's hard to imagine wh

The phrasing "Legal but non-reachable " makes clear that they use some notion of legal that differs from the normal one of reachability. It's hard to imagine what sensible notion that could be though. Something like: each side having only one king, no pawns on first/last row, at most one king in check, etc ?!