1. W. Heisenberg:Nucl. Phys.,4, 532 (1957).
2. G. KÄuén andW. Pauli :Dan. Mat. Fys. Medd.,30, no. 7 (1955).
3. We refer to Dirac’s book (P. A. M. Dirac:The principles of quantum mechanics, Cambridge, 1947), where such calculation rules are defined abstractly, without using any assumption about metric: indeed formula (8) p. 21 (A ¦A > 0 except when ¦A = 0) is not used up to Sect.8 (p. 28).