If we find cut sets together with synthon reactions that pass the connector region heuristic based on the unlabeled fragments, we start enumerating all labeled fragments. In a first step, we sort the fragments by their potential to eliminate non-matching synthon reactions. We found that sorting descending by the number of set bits in the fragment fingerprint leads to excellent results. We take the fragment f L with the most set bits in the fragment fingerprint and check if we find among all fragment fingerprints of labeled synthons any bitsets that are a superset of the fragment fingerprint of f L . We found that this heuristic works very efficiently to eliminate labeled splits that cannot be matched to any synthon reaction of the synthon space. Please note that we can again use the method for fast superset testing to evaluate this heuristic.