Editing Cahn–Ingold–Prelog priority rules (section)

====Cyclic molecules====
To handle a molecule containing one or more [[Cyclic molecule|cycles]], one must first expand it into a [[Tree (graph theory)|tree]] (called a '''hierarchical digraph''') by traversing bonds in all possible paths starting at the stereocenter. When the traversal encounters an atom through which the current path has already passed, a phantom atom is generated in order to keep the tree finite. A single atom of the original molecule may appear in many places (some as phantoms, some not) in the tree.<ref name="ciprev1982">{{cite journal |last1=Prelog |first1=Vladlmir |last2=Helmchen |first2=Guenter |title=Basic Principles of the CIP-System and Proposals for a Revision |journal=Angewandte Chemie International Edition in English |date=August 1982 |volume=21 |issue=8 |pages=567–583 |doi=10.1002/anie.198205671 }}</ref>{{rp|at=572}}