Software called legal expert systems are used around the world by private and public organizations to compute taxes. A bug in such programs can lead to tax miscalculations and heavy legal and democratic consequences. Yet, increasing evidence suggests that some legal expert systems may not meet satisfying criteria to be in compliance with the law. Moreover, they are difficult to adapt to the continuous flow of new legislation just by using traditional software development process. To prevent further software decay and reconcile these systems with the growing demand for algorithmic transparency, we argue that there is a need for a new development process for legal expert systems. As such, we present a solution built by lawyers and computer scientists : a new programming language coupled with a pair programming development process.