theorem Th27:
  OuterVx(f,n) c= UnusedVx(f,n)
proof
  let x be object;
  assume x in OuterVx(f,n);
  then ex k st x=k & k in dom f & 1 <= k & k <= n & f.k <> - 1 & f.(n+k) <> -1;
  hence thesis;
end;
