Sinking folds, the dark part is concave inward. Comparing with 37 and 39, we can see that the dark part is concave and hidden. 38 is the development of 37, and the peak of the edge contour of the deep area in 38 is the crease protrusion, so that the dark part can be recessed. You don't know if this settlement estimate is a broken line that converges at the midpoint inside. I don't know about that. You have to try it slowly, like the line in the middle of the dark part on the left. After sinking, the crease must be concave inward. Try the rest slowly. It's easy to try to get the peaks of the outline out first, and then press the dark one in the middle.