This paper shows an example of developing a fusion system in a formal framework, i.e., through the use of formal operators in the development process. Two main concepts of formal methods are theories and models. In our approach, the development of a fusion system consists of operations on theories and models. We show, on a simple example, how theories and models are combined in the process of designing a fusion system. We also compare the performance of a system developed according to our approach with a more traditional system.