Writers must ensure that all selected atoms can be deselected. Consider pyrrole, erroneously encoded with a selected nitrogen atom (e.g., "n1cccc1"). The nitrogen atom can not be pruned because of its non-zero subvalence . The DS therefore contains all five atoms and all five bonds. A perfect matching does not exist. A reader receiving such a string must report an error. To avoid this outcome, a writer can consume the subvalence by adding a virtual hydrogen (e.g., "[nH]1cccc1"). The subvalence of nitrogen in this case is zero, so the atom can be pruned. Doing so leaves a DS with four atoms, four edges, and a perfect matching. Readers will therefore consider the string valid. The larger question of gratuitous selection nevertheless remains.