This is a special merging mode that counts on newer versions of MAME not respecting the romof tags, leaving parents with files that are not actually required by the children. This new merging mode ignores those romof and cloneof tags, using devices only to add more required roms.
This is the same diffing that is used by Romba to output its diffs. It is currently untested. Basically, this will take a set of base DATs as an input, and then diffs all of the individual DATs against that combined base and then outputs accordingly.
