code: def number_of_singleton_referents(self):
          
          if self.__dict__['referential_domain']:
              ct = 0
              for i in self.iter_singleton_referents():
                  ct += 1
              return ct
          else:
              raise Exception("self.referential_domain must be defined")
docs: Returns the number of singleton elements of the referential domain that are
          compatible with the current belief state.
  
          This is the size of the union of all referent sets.