reserve x for set,
  p,q,r,s,t,u for ExtReal,
  g for Real,
  a for Element of ExtREAL;

theorem
  [.r,s.[ misses {s}
proof
  let t;
  assume t in [.r,s.[;
  then t < s by Th3;
  hence thesis by TARSKI:def 1;
end;
