Erin Sheldon and Matt Becker have requested the ability to add and subtract heavy footprints from images, while currently we can only do an insert to replace the heavy footprint wholesale.

This is fairly straightforward to implement (I have a prototype implementation at https://github.com/lsst/afw/commit/54d362337947e2fc907506b95b8c8bc0b0eed436 ), and would allow photometry to be done on images that have deblended neighbors subtracted as an alternative to the current NoiseReplacer method.