1. University of Maryland Institute for Advanced Computer Studies, A.V. Williams Building, University of Maryland; College Park Maryland 20742 U.S.A.
2. Department of Computer Science, Columbia University; 450 Computer Science Building, 1214 Amsterdam Avenue New York 10027 U.S.A.
3. Department of Botany, MRC-166, United States National Herbarium, National Museum of Natural History, Smithsonian Institution; P.O. Box 37012 Washington DC 20013-7012 U.S.A.