theorem
  Sum<*0.V,u,v*> = u + v & Sum<*u,v,0.V*> = u + v & Sum<*u,0.V,v*> = u + v
proof
  thus Sum<* 0.V,u,v *> = 0.V + u + v by Th46
    .= u + v;
  thus Sum<* u,v,0.V *> = u + v + 0.V by Th46
    .= u + v;
  thus Sum<* u,0.V,v *> = u + 0.V + v by Th46
    .= u + v;
end;
