A symbolic model is expressed using a description language, a representation of the model that is often confused with its interpretation. For example, the expression 3 + 5 may be intended to mean a particular natural number, but here is just notation for the syntactic model of the natural numbers. Strictly speaking, a description language describes both the syntactic structure of the model and its notation, i.e., the words or symbols used for the concepts in the language. As we explained in Sect. 3.3.1, we make a strict separation between structure and the notation, and we will use the term ‘model’ to refer to the structure.