code: def BC(self):
          '''
          Vertices B and C, list.
  
          '''
          try:
              return self._BC
          except AttributeError:
              pass
          self._BC = [self.B, self.C]
          return self._Bf BC(self):
          '''
          Vertices B and C, list.
  
          '''
          try:
              return self._BC
          except AttributeError:
              pass
          self._BC = [self.B, self.C]
          return self._BC
docs: Vertices B and C, list.