docs: def _mark_vertices(self, subdomain):
          """Mark faces/edges which are fully in subdomain.
          """
          if subdomain is None:
              is_inside = numpy.ones(len(self.node_coords), dtype=bool)
          else:
              is_inside = subdomain.is_inside(self.node_coords.T).T
  
              if subdomain.is_boundary_only:
                  # Filter boundary
                  self.mark_boundary()
                  is_inside = is_inside & self.is_boundary_node
  
          self.subdomains[subdomain] = {"vertices": is_inside}
          return