
theorem Th2:
  for r being object holds r is Element of RATPLUS iff r is positive Rational
  proof
    let r be object;
    hereby
      assume r is Element of RATPLUS;
      then r in the set of all r where r is positive Rational;
      then ex s be positive Rational st r = s;
      hence r is positive Rational;
    end;
    assume r is positive Rational;
    then r in the set of all r where r is positive Rational;
    hence r is Element of RATPLUS;
  end;
