
theorem FSS:
  for f be FinSequence of COMPLEX holds Sum FS2XFS f = Sum f
  proof
    let f be FinSequence of COMPLEX;
    reconsider g = FS2XFS f as XFinSequence of COMPLEX;
    Sum g = Sum XFS2FS g by XSS
    .= Sum f;
    hence thesis;
  end;
