Manuscript received January 1;
1985;
revised August 11;
1986. This work was supported in part by Nato Research Grant 280.81 and by a Doctoral Dissertation Fellowship from the University of Minnesota. G. Balbo is with the Dipartimento di Informatica;
Universita di Torino;
Torino;
Italy. S. C. Bruell is with the Department of Computer Science;
University of Iowa;
Iowa City;
IA 52242. S. Ghanta is with the Department of Computer Science;
University of Minnesota;
Minneapolis;
MN. IEEE Log Number 8718946;