reserve X,Y for set,
  x,x1,x2,y,y1,y2,z for set,
  f,g,h for Function;
reserve M for non empty set;
reserve D for non empty set;

theorem
  not {} in BOOL D
proof
  assume not thesis;
  then not {} in {{}} by XBOOLE_0:def 5;
  hence thesis by TARSKI:def 1;
end;
