 Later several proposals for dealing with this have been developed; surveys can be found in Bäuerle and Cresswell (2003), Fox and Lappin (2005), and Egré (2021). The latter authors explain that there are two strategies: the first is to introduce impossible worlds in which woodchuck and groundhog are not equivalent, and the second is to introduce an entailment relation with the property that identity does not follow from reciprocal entailment. Fox and Lappin follow the second strategy.