From the 333 prediction enumerations, only one token length could potentially give the ground truth for the missing molecule, and the other 332 enumerations are semantically meaningless predictions, and even sometimes, the SMILES syntax of the predictions is incorrect. To determine the meaningful result, RDKit API was used to convert the 333 candidate molecular SMILES strings into molecule objects, and the syntax meaningless molecules would throw error and be removed from the candidate molecules. For example, for the paracetamol synthesis reaction, only 7 out of the 333 enumeration produce SMILES syntax valid molecules, which have the token lengths of 1, 3, 4, 5, 9, 11 and 12 respectively. Among these scenarios, the 11 tokens concatenate into two molecules, with "." presented in the SMILES string, as shown in Figure .