1. J. Distler and H. Kawai, Cornell preprint CLNS 88/853.
2. F. Smirnoff and L. Takhtajan, University of Colorado preprint (March 1990).
3. J. Polchinski, University of Texas preprint UTTG-19-90.
4. N. Seiberg, Rutgers preprint RU-90-29.
5. J. Distler, Princeton preprint PUPT-1161.