Reduction Theorem

The operational semantics of the programming model —action systems in chapter 2 and object-oriented systems in chapter 3— is based on tight executions, where each action execution is completed before another one is started. This is a convenient model for understanding a program and reasoning about its properties, because an action represents an indivisible unit whose execution cannot be preempted by another. We applied induction on the number of actions in a tight execution to deduce invariant properties in chapter 5, for instance. We developed a logic to reason about tight executions in chapters 5, 6, 8, and 9, and we extend the logic for the general programming model in chapter 12.