While this illustrates the potential advantage of working with a catalyst grammar, the presented example is obviously only of a toy nature. Clearly, the elemental composition is an overly simplistic representation of a real catalyst. In the rest of this perspective, we therefore want to discuss some of the challenges and requirements for the development of a more general grammar of heterogeneous catalysts. String representations: Since formal grammars are intimately connected with strings and languages, one way forward would be to develop more useful string-based representation of catalyst materials. The difficulty therein is that (unlike the elemental composition) the three dimensional arrangement of atoms in a solid does not naturally map onto a one-dimensional string. While this is also true for organic molecules, the SMILES language uses powerful, domain-specific abstractions like chemical bonds, implicit hydrogen atoms and atom-typing to achieve this mapping. Defining such abstractions is strongly simplified by the small number of elements that are relevant in organic chemistry. Even with these advantages, non-local features like rings still cause problems with SMILES, e.g. for machine-learning applications. Unfortunately, such features are ubiquitous in solids, due to the presence of highly coordinated atoms (e.g. transition metals).