
theorem Th3:
  for y being set st [{},y] in REAL holds y <> {}
proof
  let y be set such that
A1: [{},y] in REAL and
A2: y = {};
  [{},y] in {[{},{}]} by A2,TARSKI:def 1;
  hence contradiction by A1,XBOOLE_0:def 5;
end;
