The Role of Formal Proof in Modelling Interactive Behaviour

This paper discusses and exemplifies some of the trade-offs that need to be considered when proving properties of formally modelled interactive systems. The properties we are interested in concern the usability of an interactive system, such as showing that things users want to do are easy to do. A simple model of a webbrowsing system is proposed and we show how we can expose usability problems and unjustifiable assumptions about users. We also show the difference between safety and liveness in such models and suggest that we may be able to get discount proofs with safety based models and properties, but at the cost of some loss of expressiveness and intuitive meaning.