[Dx4win] QSL Filtering Question - New Ones

[ARS NZ3O (Byron) FM29fx]

I'm probably a bit dense here, but how do I most
simply filter QSO's so I can set label=y to QSL the contacts that
I have worked but haven't yet mailed any QSL for that band/mode?

I attempted to filter
band=80m
confirm=n
but I can't figure out how to get "QSLdate=NULL"
for the filter.

73, Byron