It might be useful in debugging, but it got pretty badly in the way while
merging with PostgreSQL 8.3. We could fix it, of course, but on balance,
I don't think it's worth the effort. It's going to be a maintenance burden
going forward too, as the WATCH_* calls are scattered all over the
visibility checking code. If we need debugging code like that, we should
find a less invasive way to implement it, or submit the mechanism to
upstream so that we wouldn't need to maintain it as a diff.