Section 40.1.  Symmetries: Operations That Cancel Each Other

40.1. Symmetries: Operations That Cancel Each Other Many business transactions have natural counterparts, as there is a kind of resource-balancing process going on. Goods are traded for cash, currencies are traded based on an exchange rate, time is exchanged for consulting fees, and bookings should all expire and bought goods replaced or returned. With hiring in the RPS system, there is a balancing between the rental items going out and coming back or, occasionally, being exchanged for cash or written off because they weren't returned. So it's easy to write a program with a model for that process, involving multiple clients and rentals. Interleaving the rentals and returns is more likely to stretch the system and find errors, so we'll do that. The design of model-based test generation entails several issues about how we generate the tests, run and report results, and develop the underlying model. One approach would be to generate Fit tables in files and run Fit on them as a group. But a more direct way is to generate the tables and run them with CustomRunner. We don't want to have to manually run the tests and check the results afterward. It will be convenient to have the tests run automatically, maybe all the time. We want to be informed only when things go wrong, with sufficient information to be able to replicate any errors that are found. Finally, we want to keep the model as simple as possible, for two reasons. One is to reduce the cost of creating it. The second is to reduce the chance of finding errors in the model rather than in our system.