1. Nambu, Lectures at the Copenhagen Summer Symposium (1970).
2. T. Goto Progr. Theoret. Phys. 46 1560 (1971).
3. For earlier formulations, see also H. B. Nielsen 15th International Conference of High Energy Physics (Kiev), 1970.
4. L. Susskind Nuovo Cimento 69A 457 (1970).
5. See C. B. Thom, in Unified String Theory (edited by M. B. Green and D. Gross), World Scientific, Singapore, 1985.