This is in reference to the Negate rack extension by Tonicmint.
The description says it can produce "negative melodies" but I wanted to make sure that means the same thing that it does in music theory as I would think it would be tricky for a player device to do that. But maybe I'm wrong.
What I'm talking about is also called "chromatic inversion" of a melody.
The video by this guitar instructor explains it much better than I could.
Basically, you are reversing the direction of the Interval steps between notes. Up 3 half steps becomes down 3 half steps, followed by down a half step instead of up a half step.
I would think the device would need to know to keep the first note unchanged and then "see" the length and direction of the intervals following that note in order to alter them properly.