code: def crc(self):
          
          # first CRC the points in every entity
          target = caching.crc32(bytes().join(e._bytes()
                                              for e in self.entities))
          # add the CRC for the vertices
          target ^= self.vertices.crc()
          return target
docs: A CRC of the current vertices and entities.
  
          Returns
          ------------
          crc: int, CRC of entity points and vertices