For a selected atom, implicit hydrogen count equals subvalence minus one. This subtraction accounts for the extra valence implied by the atom's membership in the delocalization subgraph. Consider a selected carbon atom in benzene with a hydrogens attribute of Implicit. Subvalence equals two (4 -2), so implicit hydrogen count equals one (2-1). Likewise, the subvalence of a selected nitrogen atom in pyridine equals one (3 -2) so implicit hydrogen count equals zero (1 -1). Stated differently, the calculation ensures that an atom's implicit hydrogen count is identical before and after deselection.