As exemplified in Fig. , sentences can be generated from this grammar by applying the production rules (depicted as branched arrows) in a sequential manner. Starting from the non-terminal symbol S, only a single production rule is available (S → NP Pre CN). The final sentence is reached when no non-terminal symbols are left and therefore no more production rules can be applied. Importantly, this sentence is only one of many that can be generated by the grammar in Fig. . In Table , some other examples are shown, along with random sentences constructed by combining arbitrary words from the dictionary Σ. These sentences also serve to illustrate the distinction between syntax and semantics: The grammar generates sentences that are syntactically valid. This does not mean that these sentences are necessarily meaningful (i.e. semantically valid). We are however much more likely to generate a meaningful sentence with the grammar than with the random generator, which produces Table . Example sentences generated with the grammar in Fig. and by randomly combining five words from the corresponding dictionary. The grammar leads to syntactically valid sentences but not necessarily semantically valid (i.e. meaningful) ones. The random generation meanwhile produces complete gibberish. Catalyst grammars: While this may seem far removed from heterogeneous catalysis, string-based representations and concomitant grammars have actually already found wide application in the not so distant field of organic chemistry, e.g. in the form of SMILES strings or the more recent SELF-IES grammar. The dictionaries of these chemical languages consists of atoms and bonds that are combined to form strings representing molecules. Importantly, the corresponding syntax imposes physical and chemical constraints into what kinds of molecules can be formed. For instance, the SELFIES grammar is constructed such that all generated strings by definition fulfill the valence rules of organic chemistry. Despite their undisputed importance in organic chemistry, strings and grammars are much less developed for the inorganic and condensed-phase systems of interest in heterogeneous catalysis, however. Arguably, this results from the much higher complexity and variability of the corresponding extended materials. Before entering a more differentiated discussion onto this matter, let us first further motivate why striving for such grammars could be a worthwhile endeavor. To this end, we consider the simple toy problem of finding stable ionic material compositions out of the 28 main group elements in periods 2-5 and groups 1-17 (i.e. from Lithium to Iodine). For simplicity, we assume that each element only occurs in oxidation states that lead to the closest noble gas configuration (e.g. Li +1 , Mg +2 , Cl -1 , etc.). To screen potential catalysts from these elements we could consider as a simple rule only those binaries of the type AB with balanced charges (e.g. Na +1 Cl -1 , Mg +2 O -2 , Ga +3 As -3 , etc). Unfortunately, this leads to a disappointingly low total of 55 possible materials and shows that vaster spaces need to be spanned to possibly identify new promising materials in the screening. This can be achieved by expanding our search space up to quaternary compositions and considering all combinatorial possibilities, which leads to a much larger library of over 600,000 candidates. However, these mostly correspond to unlikely (electronically unbalanced) compositions such as Na + O -2