Without going into detail here, suffice it to say that atomic number averaging is a difficult procedure to describe or implement, other than the case of all-carbon neutral species, for which it is unnecessary, and some simple heterocyclic systems, such as pyridine derivatives. Thus, consider substituted pyridine VS032/ VS033, Figure . Notice that without this consideration, Rule 1a gives two different results in this case, depending upon the choice of Kekulé structure. The correct result, S, derives from assigning the duplicate node for the aromatic nitrogen an averaged atomic number of 6.5, which loses to the unaveraged atomic-number 7 duplicate node of the imine nitrogen.