I don't have many experiences with the Cog branch but the "old" VM build system allows for a separate build folder and thus cleaning is very easily done by removing all files and subfolders. I haven't seen this for the Cog VM yet but I only compiled it once for my Mac and it went flawlessly.That's possible. Unfortunately there is no 'clean' target for make. I'll remove the .o files for vm.a by hand and retry it.