theorem Th2:
  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;
