This is a follow-up of sorts to id:"8761ycc19t.fsf@qmul.ac.uk", where Mark suggested that the part handling commands could all use the correponding mm-* functions. I ran with the idea and wound up with this series, which, in addition to standardizing on the mm-* functions for everything and simplifying the implementation overall, decouples the part commands from part buttons, which removes an entire layer from the implementation and adds the ability to invoke part commands with point anywhere in a part (something I often find myself wanting).