1. For terminology, nomenclature and notations not defined in this paper, see G. Boolos,The Unprovability of Consistency, Cambridge University Press, 1979, where the system GL is consideredin extenso, along with several other systems of modal propositional logic.
2. R. Magari, ?Primi risultati sulla variet� di Boolos?,Bollentino U.M.I. (6)1-B (1982), 359?367.
3. David Lewis, ?Intensional logics without iterative axioms?,Journal of Philosophical Logic 3 (1974), 457?466.
4. J. F. A. K. van Benthem, ?Two simple incomplete modal logics?,Theoria 44 (1978), 25?37, and ?Syntactic aspects of modal incompleteness theorems?,Theoria 44 (1979), 63?77.
5. L. Henkin, ?A problem concerning provability?,Journal of Symbolic Logic 17 (1952), 160.