I personally think that there should be an unmix tool.  It would take all the elements in the area of the cursor and put them into (somewhat) equal squares, so it would be easier to delete certain elements from something that was mixed up previously, instead of it taking up to a few hours to delete clumps of material with shift-ctrl, followed by deleting individual pixels, and then letting them fall and mix them up again, only to find out that you only deleted a quarter of the pixels you were trying to get rid of, and repeating the process over and over again, and then realizing there were other elements you were trying to get rid of.