Each "@@@" symbol was converted to the multiple of the mask symbol "<mask>" based on the number of tokens included in the hidden molecule. In this way, instead of dynamically randomisng 15% of the mask, the masks in the test dataset were tailored to mask an entire The hidden molecule mask method was initially also considered as a regularised mask method to mask molecules during training instead of the dynamic mask approach. However, such a mask method hides a molecule at a centralised segment (i.e. one single token) of the reaction SMILES. By no means could the mask method learn bond breaks and recombination from the reaction SMILES, and it was hard to pick the semantic context among the molecules. Therefore, a dynamic mask approach was implemented.