Wick-pairing multiplicity for a zeta zeta-prime-squared vertex ($3\times 2=6$)

Three external fields can be joined to the three cubic vertex fields in six connected Wick contractions. There are three choices for the external field at the undifferentiated slot and two exchanges of the differentiated slots. Three cyclic terms therefore require an explicit factor of two. Ordered contractions with an earlier vertex use complex-conjugated mode functions.