theorem Th3:
  t in [.r,s.[ iff r <= t & t < s
proof
  hereby
    assume t in [.r,s.[;
    then ex a st a = t & r <= a & a < s;
    hence r <= t & t < s;
  end;
  t is Element of ExtREAL by XXREAL_0:def 1;
  hence thesis;
end;
