Another specially treated system are rings of size three containing one hetero atom that share the bond opposite to the hetero atom with another ring (Figure ). When rings like this are removed, the shared bond is turned into a double bond to produce the precursor structure the hetero atom was most likely added to. This special case is described in the first ring removal prioritisation rule by Schuffenhauer et al. but is part of the general ring removal routine of Scaffold Generator. This deviation from the original implementation does not influence the parent scaffold generation according to the scaffold tree prioritisation rules but is important to note for the enumerative generation of all possible parent scaffolds (see below).