This research was supported at SRI htternational by the Defense Advanced Reseat~ch Projects Agency under contract N00039-79-C-0118 ~¢ith the Naval Electronic Systems Commaw t The views and conchtsions contained in this document are those of the author and should not be interpreted as representative of the official policiex either expressed or bnplied, of the Defense Advanced Research Projects Agency, or the U. S. Goverttment. The author is gratefid to Barbara Grosz, Gary ttendrix and Terry Winograd for comments on an earlier draa of this paper. .
