reserve s for set,
  i,j for Nat,
  c,c1,c2,c3 for Complex,
  F,F1,F2 for complex-valued FinSequence,
  R,R1,R2 for i-element complex-valued FinSequence;

theorem
  Sum(R1 - R2) = Sum R1 - Sum R2
proof
  thus Sum(R1 - R2) = Sum R1 + Sum -R2 by Th40
  .= Sum R1 - Sum R2 by Th39;
end;
