Returns the size of the belief state.

        Initially if there are $n$ consistent members, (the result of `self.number_of_singleton_referents()`) 
        then there are generally $2^{n}-1$ valid belief states.

def size(self):
        """ Returns the size of the belief state.

        Initially if there are $n$ consistent members, (the result of `self.number_of_singleton_referents()`) 
        then there are generally $2^{n}-1$ valid belief states.
        """
        n = self.number_of_singleton_referents()
        targets = list(self.iter_referents_tuples())
        n_targets = len(targets)
        if n == 0 or n_targets == 0:
            return  0

        #if len(self.__dict__['deferred_effects']) != 0:
        #    return -1 
        size1 = len(list(self.iter_referents_tuples()))
        tlow, thigh = self['targetset_arity'].get_tuple()
        clow, chigh = self['contrast_arity'].get_tuple()
        return size1