High frequent expressions were usually present within the molecule representations of the reaction SMILES, i.e. the splitting molecule symbol "." and the splitting reactant-product symbol ">>" do not exist in the tokens. However, in several cases, tokens were formed across the molecule representations, since the BPE tokenisation method recognises SMILES strings of "part of a molecule's SMILES + . + part of another molecule's SMILES" as high frequent expressions, such as "+].[" and ".[".