In masking we should have a Negative/Positive Effect setting.
This Negative/Positive settings should be a checkbox and simply when enabled the area/pixels of the photo affected by the Mask would be turned into Negative or Positive (in case one is working on a film scan)
This simple setting would be very useful for those that work with film scans and as well for all creative uses.
I am perfectly aware that we can already turn a photo into Negative/Positive using a Point Curve but thanks to the Positive settings in Masking we would be able to do it also locally so only on a specific part of the image.
The advantage of using a mask to turn complete/partially the image into a Negative/Positive is also that Point Curve would be kept linear not inverted and for most users it's easier to work like that.