Session 5. Changes to the area at top left of this detail are subtle, but they solidify, add detail, smooth gradation. More could happen here. More can always happen. Always more after that. But at this point, I don't think anything needs to.
I sketch in-drawing by pulling shapes from the charcoal and graphite midtones with a sharp kneaded or stick eraser. Shapes are quick this way, and I don't get pencil marks/scorings that may be problematic later. The roughed-in bits at bottom center aren't there yet. Their shape and flow seem to work well with those at bottom right. But I expect they'll change.