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,s}
proof
  let t;
  assume
A1: t in ].r,s.[;
  then
A2: r < t by Th4;
  t < s by A1,Th4;
  hence thesis by A2,TARSKI:def 2;
end;
