1. See Ref.’s 2–6.
2. D. Amati, E. Leader and B. Vitale, Nuovo Cimento 17: 68 (1960).
3. G. Epstein and B. McKellar, Lettere al Nuovo Cimento 5: 807 (1972).
4. G. Bohannon and P. Signell, Phys. Rev. D10: 815 (1974).
5. M. Lacombe, B. Loiseau, J. Richard, R. Vinh Mau, J. Cote, P. Pires, and R. de Toureil, Orsay Preprint IPNO/TH 78–46, Nov. 1978 and private communication from R. Vinh Mau.