1. Cf. D. Føllesdal and R. Hilpinen, ‘Deontic Logic: An Introduction’ in Hilpinen 1971.
2. Cf. Alchourrón-Bulygin 1976,1977,1978. This problem has been already seen by Cornides, though he seems not to give it much importance. Cf. Cornides 1969, p. 1241.