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2. Ph. Dwinger, Ideals in generalized Post algebras, ibid. 17 (1969), pp. 483?486.
3. Ng. Cat Ho, Generalised Post algebras and their application to some infinitary many-valued logics, Dissertationes Mathematicae CVII (1973), pp. 1?76.
4. Ng. Cat Ho, A relational model of databases with context dependent null values, submitted for publication.
5. H. Rasiowa, On generalized Post algebras of order ?+ and ?+-valued predicate calculi, Bulletin de l'Acad�mie Polonaise des Sciences. S�rie des Sciences Math�matiques, Astronomiques et Physiques 21 (1973), pp. 209?219.