> the parts that need rebuilding are the parts that read those files when they were last built... ...and the transitive closure of those parts, which is where

> the parts that need rebuilding are the parts that read those files when they were last built... ...and the transitive closure of those parts, which is where things get complicated. It may be that the output didn't actually change, so you can prune the graph there with some smarts. It may be that the thing changed was a tool used in many other rules. And you have to know the complete set of outputs and inputs of every action. And on and on.