*This article originally appeared in Science , vol. 185 , 1974. The research
was supported by the Advanced Research Projects Agency of the
Department of Defense and was monitored by the Office of Naval
Research under contract N 00014 - 73 - C- 0438 to the Oregon Research
Institute, Eugene. Additional support for this research wass r" 0 %" wid
provided by the Research and Development Authority of the Hebrew
University, Jerusalem, Israel.