Query for First Parent Ancestors of commit

[joshuabalentine-PCC]

My eventual goal is to get a query to show me commits that have been merged into branch A but not into branch B, preferably without the direct line of branch A.

The core of it only(A, B) but I'd like to exclude the merge commits directly on A which should be the line of the first parents of A. I can't just use only(A, B) - merges() because that would exclude any merge of upstream branches into branch B.