code: def get_face_mask(self, subdomain):
          
          if subdomain is None:
              # https://stackoverflow.com/a/42392791/353337
              return numpy.s_[:]
  
          if subdomain not in self.subdomains:
              self._mark_vertices(subdomain)
  
          # A face is inside if all its edges are in.
          # An edge is inside if all its nodes are in.
          is_in = self.subdomains[subdomain]["vertices"][self.idx_hierarchy]
          # Take `all()` over all axes except the last two (face_ids, cell_ids).
          n = len(is_in.shape)
          is_inside = numpy.all(is_in, axis=tuple(range(n - 2)))
  
          if subdomain.is_boundary_only:
              # Filter for boundary
              is_inside = is_inside & self.is_boundary_facet
  
          return is_inside
docs: Get faces which are fully in subdomain.