Several of the generalization algorithms I am working on are based on tiles. They collect all features inside a tile boundary and do something with them. This means that whenever something changes, we have to figure out which tiles are affected and re-process them. Luckily a very similar functionality is also needed for generating lists of tiles that need to be re-rendered. And support for that has been in osm2pgsql for a long time. We just need some additions to be able to re-purpose that code.