theorem Th19:
  for f being real-valued FinSequence st Sum f <> 0 holds Sum (f (/) Sum f) = 1
  proof
    let f be real-valued FinSequence;
    Sum (f (/) Sum f) = Sum f / Sum f by RVSUM_1:87;
    hence thesis by XCMPLX_1:60;
  end;

Lm6:
