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

theorem
  ].r,s.] misses {r}
proof
  let t;
  assume t in ].r,s.];
  then r < t by Th2;
  hence thesis by TARSKI:def 1;
end;
