1. Distributive Lattices;Balbes,1974
2. Admissible Sets and Structures;Barwise,1975
3. Models and Ultraproducts;Bell,1969
4. A note on modal formulas and relational properties;van Benthem;J. Symbolic Logic,1975
5. Modal formulas are either elementary or not ΣΔ-elementary;van Benthem;J. Symbolic Logic,1976