1. J. E. Moyal, Proc. Cambr. Phil. Soc. 45, 99 (1949).PCPSA40068-6735
2. F. Bayen, M. Flato, C. Fronsdal, A. Lichnerowiz, and D. Sternheimer, Ann. Phys. 111, 111 (1977). See Sec. 1.b.ANPYA20003-3804
3. The concepts are the same as in Ref. 2, but they are approached from another direction. In Ref. 2 we dealt with a purely classical system and attempted to identify the algebra of distinguished observables by intuitive physical arguments. We then proposed a quantization procedure that would in effect make this the algebra of distinguished observables in the sense of Definition 3.
4. Ref. 2, Sec. I.c.
5. J. M. Souriau, Structure des systemés dynamiques (Dunod, Paris, 1970).