Yeah, the middle part that you're working on, since there's no elbow room in there. I'm imagining like a dremmel tool or something. Also thinking about the internal corners in some ancient structures, which seem impossible, but idk.
Discussion
Ya basically I have an electric die grinder with diamond bits. It’s long enough to reach in there and I just have to go slow and steady taking stone away till I get that corner. Then follow it up with a flat diamond file