reserve Omega for non empty set;
reserve Sigma for SigmaField of Omega;
reserve S for non empty Subset of REAL;
reserve r for Real;
reserve T for Nat;
reserve I for TheEvent of r;

theorem Th72:
  for r being Real holds [.r,+infty.] is Event of Ext_Borel_Sets
proof
  let r be Real;
  Intersection ext_half_open_sets(r) is Element of Ext_Borel_Sets by Th50;
  hence thesis by Th60;
end;
