Nelson, Philip C2023-05-232023-05-231988-03-012017-04-16https://repository.upenn.edu/handle/20.500.14332/43295Talk presented a.t the Workshop on Strings and Riemann Surfaces, Mathematical Sciences Research Institute, Berkeley, March 1986.Electronic version of an article published as [International Journal of Modern Physics A, 3, 3, 1988, 585-590] [10.1142/S0217751X88000230] © [copyright World Scientific Publishing Company]Physical Sciences and MathematicsPhysicsPresentation