docs: def iter_singleton_referents_tuples(self):
          """
          Iterator of all of the singleton members's id number of the context set.
  
          NOTE: this evaluates entities one-at-a-time, and does not handle relational constraints.
          """
          try:
              for member in self.__dict__['referential_domain'].iter_entities():
                  if self['target'].is_entailed_by(member) and (self['distractor'].empty() or not self['distractor'].is_entailed_by(member)):
                      yield member['num'].low
          except KeyError:
              raise Exception("No referential_domain defined")