reserve X for set;
reserve UN for Universe;

theorem Th90:
  for X being set for S being Sequence st dom S = NAT holds
  last S = S.NAT
  proof
    let X be set;
    let S be Sequence;
    assume dom S = NAT;
    then union dom S = NAT by ORDINAL1:def 6;
    hence thesis by ORDINAL2:def 1;
  end;
