Rule 4b is by far the most difficult rule to comprehend and implement. One simplification is that although Rule 4b, as stated in BB 2013, refers to all possible mixes of R/S, M/P, and seqCis/seqTrans descriptors, for implementation purposes, all auxiliary descriptors can be normalized by labeling them either R or S. For example, any of R, M, or secCis can be assigned R for the purpose of processing Rules 4b. In this way, all discussion can be expressed in terms of "equal" or "not equal" to a reference R or S, rather than "like" vs. "unlike". It is critical that an implementation assign auxiliary descriptors involving double bonds --seqCis/seqTrans and M/P -to the sp 2 node closer to the root (or, alternatively, to both nodes equally). Otherwise the second phase of Rule 4b may fail.