Iterator of all of the singleton members of the context set.

        NOTE: this evaluates entities one-at-a-time, and does not handle relational constraints.

def iter_singleton_referents(self):
        """
        Iterator of all of the singleton members 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'], member
        except KeyError:
            raise Exception("No referential_domain defined")