theorem Th4:
  Sum(L1 - L2) = Sum(L1) - Sum(L2)
proof
  thus Sum(L1 - L2) = Sum(L1) + Sum(- L2) by Th1
    .= Sum(L1) + (- Sum(L2)) by Th3
    .= Sum(L1) - Sum(L2) by RLVECT_1:def 11;
end;
