Fig. shows a simple de-esterification as an example. Here, only one missing fragment is detected. Because the carbon-oxygen bond is part of an ester group, an expand rule adds the missing oxygen atom to the reaction. In the second step, a merge rule connects this oxygen with a single bond to the open boundary on the identified fragment, creating the missing acetic acid. The resulting reactions is carbon balanced but unbalanced overall. The rule-based method described in Section 2.2 is now applicable to add the missing water molecule to the reactants.