This is a simple trick that shows the utilization of “surface split” component in Grasshopper. It is used to detect inner regions of any given two-dimensional linework, resembling the hatch boundary detection of AutoCAD. There is no hatch component in Grasshopper but maybe this could be used as a starting point. Here is the simple definition if you want to try: [GHX: 0.9.0056]. I used “project” component to quickly understand which of the trimmed surfaces is inside. “Point in curves” component also gives the same solution.