1. Boolos. G., ?Extremely undecidable sentences?, forthcoming.
2. Boolos, G., ?Provability in arithmetic and a schema of Grzegorczyk?, to appear in Fundamenta Mathematicae.
3. Boolos, G., The Unprovability of Consistency, Cambridge, 1979.
4. Goldblatt, R., ?Arithmetical necessity, provability, and intuitionistic logic?, to appear in Theoria.
5. Segerberg, K., An Essay in Classical Modal Logic, Uppsala, 1971.