docs: def is_contradictory(self, other):
          """ Returns True if the two DictCells are unmergeable.  """
          if not isinstance(other, DictCell):
              raise Exception("Incomparable")
          for key, val in self:
              if key in other.__dict__['p'] \
                      and val.is_contradictory(other.__dict__['p'][key]):
                  return True
          return False