code: def is_contradictory(self, other):
          
          for (s_key, s_val) in self:
              if s_key in other and s_val.is_contradictory(other[s_key]):
                  return True 
          return False
docs: Two beliefstates are incompatible if the other beliefstates's entities 
           are not consistent with or accessible from the caller.
           
          Note: this only compares the items in the DictCell, not `pos`,
          `environment_variables` or `deferred_effects`.