docs: def number_of_singleton_referents(self):
          """
          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.
          """
          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")