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`.

def is_contradictory(self, other):
        """ 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`.
        """
        for (s_key, s_val) in self:
            if s_key in other and s_val.is_contradictory(other[s_key]):
                return True 
        return False