code: def CB(self):
          '''
          Vertices C and B, list.
  
          '''
          try:
              return self._CB
          except AttributeError:
              pass
          self._CB = [self.C, self.B]
          return self._Cf CB(self):
          '''
          Vertices C and B, list.
  
          '''
          try:
              return self._CB
          except AttributeError:
              pass
          self._CB = [self.C, self.B]
          return self._CB
docs: Vertices C and B, list.