 reserve Omega, Omega2 for non empty set;
 reserve Sigma, F for SigmaField of Omega;
 reserve Sigma2, F2 for SigmaField of Omega2;

theorem Th8:
for k1,k2 being Real holds [.k2,k1.] is Element of Borel_Sets
proof
  let a,b be Real;
  Intersection half_open_sets(b,a) = [.b,a.] by Th6;
  hence thesis by Th5;
end;
