Stereochemical canonical value

If there are at least two nonterminal double bonds in the molecule the algorithm continues and calculates a "stereochemical canonical value", in which the initial canonical value is a number meaning cis or trans with respect to a double bond. If the atoms concerned are still equivalent with respect to the steroeochemical canonical value, then they are truly equivalent unless they are both chiral. This step is required to reveal the inequivalence of atoms 1 and 2 of the Figure below. 

If all the pairs of S are still equivalent then go to step 7. Otherwise remove the inequivalent pairs from S and continue to calculate stereochemical canonical values and check for inequivalences of pairs in S until the length of S no longer decreases. 

Systematic names of compounds have been used in many systems to identify and describe their chemical structures. Until recently, however, those names were mere identifiers of the structures and could not be decoded into connection tables. Stereochemical naming schemes have used the Cahn-Ingold-Prelog (CIP) priority system extensively. As it associates a label (R, S, r, s, 0, or E, Z, e, z, see structures 3-5 for some examples) with an atom or bond, it has been used to code stereochemical information as node and edge attributes in standard connection tables. The labels were manually assigned and could be used in canonical numbering methods because their values did not depend on the particular numbering. 

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