code: def BA(self):
          '''
          Vertices B and A, list.
  
          '''
          try:
              return self._BA
          except AttributeError:
              pass
          self._BA = [self.B, self.A]
          return self._Bf BA(self):
          '''
          Vertices B and A, list.
  
          '''
          try:
              return self._BA
          except AttributeError:
              pass
          self._BA = [self.B, self.A]
          return self._BA
docs: Vertices B and A, list.