Same field, different table. Return differences

Hi,

I am designing a report using the Rows repeater.

It's pulling from the Purchase Line table as the primary source.

But, I want to link to this record in the Purchase Header table and only return results where the Dimension Set ID field is not the same.

I would expect something like this to work, but Jet isn't having it; the nested NL formula is an "invalid filter."

=NL("Rows","Purchase Line",,"Dimension Set ID","<>NL(""First"",""Purchase Header"",""Dimension Set ID"",""No."",NF(,""Document No.""))"

Note: I've had to do the nested NL as Quotes because of the <> operator; I imagine if I wanted to return = matches, I wouldn't use the Quote.

Is this achievable in a single formula?

Thanks