1. G. Birkhoff and J. von Neumann,Ann. Math. 37, 823 (1936).
2. C. Piron,Helv. Phys. Acta 37, 439 (1964).
3. J. M. Jauch,Foundations of Quantum Mechanics (Addison-Wesley, Reading, Mass., 1968).
4. S. Watanabe,Knowing and Guessing (Wiley-Interscience, New York, 1969).
5. C. Piron,Foundations of Quantum Physics (W. A. Benjamin, Reading, Massachusetts, 1976).