theorem
  Sum<* u,v,w *> = Sum<* u *> + Sum<* v *> + Sum<* w *>
proof
  thus Sum<* u,v,w *> = u + v + w by Th46
    .= Sum<* u *> + v + w by Lm6
    .= Sum<* u *> + v + Sum<* w *> by Lm6
    .= Sum<* u *> + Sum<* v *> + Sum<* w *> by Lm6;
end;
