Abstract. We present Unit-B, a formal method inspired by Event-B and UNITY, for designing systems via step-wise refinement preserving both safety and liveness properties. In particular, we introduce the notion of coarse- and fine-schedules for events, a generalisation of weak- and strong-fairness assumptions. We propose proof rules for reasoning about progress properties related to the schedules. Furthermore, we develop techniques for refining systems by adapting event schedules such that liveness properties are preserved. We illustrate our approach by an example to show that Unit-B developments can be guided by both safety and liveness requirements.
Keywords: progress properties, refinement, fairness, scheduling, Unit-B.