theorem Th26:
  UnusedVx(f,n) c= Seg n
proof
  let x be object;
  assume x in UnusedVx(f,n);
  then ex i st x=i & i in dom f & 1 <= i & i <= n & f.i <> - 1;
  then x in {k where k is Nat: 1 <= k & k <= n };
  hence thesis by FINSEQ_1:def 1;
end;
