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2. R. Delbourgo, Abdus Salam and J.Strathdee, Nuovo Cimento, 49., 593 (1967).
3. See, for example, J. Scherk and H. J. Schwarz, Phys. Letters, 52B, 347 (1974).
4. C. J. Isham, Abdus Salam and J. Strathdee, Phys. Rev., D8, 2600 (1973)
5. Abdus Salam, Proceedings of the First Decade of the Coral Gables Conference on Fundamental Interactions, 22-26 January, 1973 ( Plenum Press, New York 1974 ),p. 55.