reserve n,k,b for Nat, i for Integer;

theorem Th6:
  for F being XFinSequence
  for X being set
  holds dom SubXFinS(F, X) = dom Sgm0(X /\ dom F)
  proof
    let F be XFinSequence;
    let X be set;
    len SubXFinS(F, X) = card (X /\ Segm(len F)) by AFINSQ_2:36
    .= card Sgm0(X /\ dom F) by AFINSQ_2:20;
    hence thesis;
  end;
