i suppose it will just take the first color that it can find which is croppable, starting at the upper left.
compare: Tools.PV(Image.Image(80,80,0,0,0)->box(10,10,70,70,255,0,0)->box(70,0,80,80,0,255,0)->autocrop()->autocrop()); and Tools.PV(Image.Image(80,80,0,0,0)->box(10,10,70,70,255,0,0)->box(70,0,80,80,0,255,0)->autocrop()->autocrop()->autocrop());