reserve n for Nat,
  k for Integer;

theorem Th19:
  for p,q,r being complex-valued FinSequence holds
  Sum (p ^ q ^ r) = (Sum p) + (Sum q) + (Sum r)
proof
  let p,q,r be complex-valued FinSequence;
  thus Sum (p ^ q ^ r) = (Sum (p ^ q)) + (Sum r) by RVSUM_2:32
    .= ((Sum p) + (Sum q)) + Sum r by RVSUM_2:32;
end;
