1. See the last parts of my paper Quantifiers in Deontic LogicSocietas Scientiarum Fennica, Commentationes Humanarum Litterarum1957 23 4 The objectionable axioms include (Op& (p⊃Og)) ⊃Ogwhich was in effect adopted by both A.N. Prior in the first edition of his Formal Logic, Oxford, 1957, and earlier by E. Mally in hisGrundgesatze des Sollens(Graz, 1926). Yet it is easily seen to be unacceptable whereasO[(Op& (p⊃Og)) ⊃Og] can be shown to be valid.
2. 1962.Knowledge and Belief: An Introduction into the Logic of the Two Notions, 71–74. Ithaca, N.Y.: Cornell University Press. e.g. 77–82, 89, 122–123, and 137–138
3. For a brief discussion of what could be done along these lines for philosophical purposes, see my paper A Program and a Set of Concepts for Philosophical LogicThe Monist1967 51 1 For a survey of the foundational aspects of model theory, see Andrzej Mostowski,Thirty Years of Foundational Studies(Acta Philosophica Fennica vol. 17), Basil Blackwell, Oxford, 1966, ch. 3, ch. 13, and ch. 14. See also J. W. Addison, L. Henkin and A. Tarski,The Theory of Models, Proceedings of the 1963 International Symposium at Berkeley, North-Holland Publishing Company, Amsterdam, 1965 (with a bibliography).