The NP(Difference) function should be able to be placed in its own cell, like NP(Union).  The first filter on one line, second filter on second line, NP(Difference) on third line, NL(Rows) on fourth line. 

NP(Difference) is very difficult to use because it now has to be embedded in the NL(Rows) line.  Even as an experienced user i have to search how to do it and i come upon the same frustrations having been expressed by many other users.  And a lot of your help desk time taken up answering them.

At least improve the documentation on this topic if you don't intend on changing it.