However, from an implementation perspective, simplified digraphs must be used with caution. In complex examples, for instance, a critical aspect of the algorithm must be finding the first difference in two ligands, and this may not be obvious from a simplified digraph. For example, there is no rule that "real atoms have higher priority than duplicate nodes." This is intentional. While generally true, this statement hides the fact that duplicate nodes lose to their real counterparts only in the next higher sphere, where their associated phantom atom, with atomic number zero, always loses to any real atom. That being the case, some other consideration may be missed. An example of a compound for which proper consideration in this regard is required is the bicyclic alkene shown in Figure (VS172). Naïve application of the pseudo-rule "real atoms have higher priority than duplicate nodes" leads to a 1R descriptor rather than correct 1S assigned in accord to Rule 1b. Sequence Rules and Subrules. While BB 2013 specifies that each of Rules 1-5 must be applied "exhaustively in the order given," it may not be clear what exactly "exhaustively" means or how this relates to subrules such as 1a and 1b. In fact, each subrule must be applied sequentially.