That doesn't necessarily follow -- it's tempting because it seems obvious. If X + Y = Z, X on surface + Y = Z _must_ follow, because "on surface" was just a h

That doesn't necessarily follow -- it's tempting because it seems obvious. If X + Y = Z, X on surface + Y = Z _must_ follow, because "on surface" was just a hidden term in X + Y = Z anyway...right? But, both biology and fabrics have a lot of hidden surface (pun intended :P)