code: def referenced_vertices(self):
          
          # no entities no reference
          if len(self.entities) == 0:
              return np.array([], dtype=np.int64)
          referenced = np.concatenate([e.points for e in self.entities])
          referenced = np.unique(referenced.astype(np.int64))
  
          return referenced
docs: Which vertices are referenced by an entity.
  
          Returns
          -----------
          referenced_vertices: (n,) int, indexes of self.vertices