In this article we present a process algebra in which the behaviour in the absence of certain actions can be specified. Processes of the form represent a behaviour which is specified by but only in an environment which cannot perform any action in S. If the environment can perform an action in S, the process is suspended. This is useful in specifying priority, time outs, interrupts etc. We present a few examples which illustrate the use of the extended calculus. A bisimulation relation induced by a labelled transition system is then considered. We present a few properties which form the basis for a sound and complete axiomatisation of a bisimulation equivalence relation. This requires an extension of the syntax. This is because the absence of information from the environment used in the operational semantics is captured syntactically. A comparison with other approaches is presented.

process algebra, absence of information, bisimulation