Priority and Abstraction in Process Algebra

More than 15 years ago, Cleaveland and Hennessy proposed an extension of theprocess algebra CCS in which some actions may take priority over others. Thetheory was equipped with a behavioral congruence based on strong bisimulation.This article gives a full account of the challenges in, and the solutions employedfor, defining a semantic theory of observation congruence for this process algebra.A full–abstraction result is presented whose proof relies on a novel approach basedon successive approximations for identifying the largest congruence contained inan intuitive but näıve equivalence. Prioritized observation congruence is also char-acterized equationally for the class of finite processes, while its utility for systemverification is demonstrated by an illustrative example.