Verification Diagrams Revisited: Disjunctive Invariants for Easy Verification

I describe a systematic method for deductive verification of safety properties of concurrent programs. The method has much in common with the “verification diagrams” of Manna and Pnueli [17], but derives from different intuitions. It is based on the idea of strengthening a putative safety property into a disjunction of “configurations” that can easily be proved to be inductive. Transitions among the configurations have a natural diagrammatic representation that conveys insight into the operation of the program. The method lends itself to mechanization and is illustrated using a simplified version of an example that had defeated previous attempts at deductive verification.