This report describes research done at the Artificial Intelligence Labors. tory of the Mss:wJmsetts Inst.:tute of Technology. S.pport for the Laboratory's :rtifici.'d intel!igencc research h,a been provided in part by the Advanced Research Projects Agency of the Deparmau of Defense uudcr Office of NavM Rescch coattact N00014-80-C-0505. This is a revised wwsiou of a Ph.D. thesis propo submitted to the Department of Elcctric Engineering and C.mputer Scieuce on Februa 28, 1984. Support for the author' graduate studies hp_s been provided by the Fannie md John Hertz Foundation