theorem Th8:
  for W holds { V : V in FX & V meets W} c= FX
proof
  let W;
  now
    let Y be object;
    assume Y in { V : V in FX & V meets W };
    then ex V st Y = V & V in FX & V meets W;
    hence Y in FX;
  end;
  hence thesis;
end;
