**VAX, UNIBUS, and MASSBUS are trademarks of Digital Equipment Corporation.

* UNIX is a trademark of Bell Laboratories.

***This work was supported under grants from the National Science Foundation under grant MCS80-05144, and the Defense Advance Research Projects Agency (DoD) under Arpa Order No. 4031 monitored by Naval Electronic System Command under Contract No. N00039-82-C-0235.