The pick first merge code allows you to select the first available entry.  It is a list of options separated by the vertical bar:

Syntax: [|<field definition1>|<field definition1>|...|<field definitionn>|<rest of field definition>]

For example   [|\Defendant#2|\Defendant#1|Name#0] would return the name of defendant #2, if defendant #2 exists, otherwise it would return the name of defendant #1.