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