0. Preliminaries....................................................................... 7 1. Adding propositional connectives to $L_ω_1_ω$............... 8 2. The propositional part of $L_ω_1_ω$ (S)............................. 10 3. The operation S and the Boolean algebra $B_S$............... 11 4. General model-theoretic properties of $L_ω_1_ω$(S)...... 17 5. Hanf number computations...................................................... 22 6. Negative results for $L_ω_1_ω$(S)........................................ 27 7. Proposition al extensions of $L_ω_1_ω$ a in the constructible universe...................................................... 34 8. The Souslin connective.............................................................. 44 9. Concluding remarks................................................................... 49 References....................................................................................... 53
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