code: def append(a, vancestors):
      
      add = True
      for j, va in enumerate(vancestors):
          if issubclass(va, a):
              add = False
              break
          if issubclass(a, va):
              vancestors[j] = a
              add = False
      if add:
          vancestors.append(a)
docs: Append ``a`` to the list of the virtual ancestors, unless it is already
      included.