Question on interface integrals at material boundaries in DG formulation

The first version produces 0 element-matrices, and tries to assemble them. But the global matrix has no entries allocated, and thus throws an exception.

The translate to dx/ds syntax should be We want a loop over boundary elements, no need for volume elements and thus no skeleton flag.

Thanks again for the clarification.
Previously, I used ds(skeleton=True,...), it gives me an error.
Now, I tried the same thing in 1D, and it seems .Trace() is not properly implemented yet for 1D Facet Dofs. It gives me this error message:

netgen.libngpy._meshing.NgException: FacetFESpace::GetSFE: unsupported element Point

Here is the code produce the error message