Degano, Pierpaolo and De Nicola, Rocco and Montanari, Ugo  Partial orderings descriptions and observations of nondeterministic concurrent processes.   In:  Linear Time, Branching Time and Partial Order in Logics and Models for Concurrency (REX Workshop 1988).   Lecture Notes in Computer Science, 354 .  Springer, pp. 438-466.  ISBN 3-540-51080-X      (1988)