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

theorem Th184:
  r < s implies [.r,t.] \ [.r,s.[ = [.s,t.]
proof
  assume that
A1: r < s;
  let p;
  thus p in [.r,t.] \ [.r,s.[ implies p in [.s,t.]
  proof
    assume
A2: p in [.r,t.] \ [.r,s.[;
    then
A3: not p in [.r,s.[ by XBOOLE_0:def 5;
A4: p <= t by A2,Th1;
    p < r or s <= p by A3,Th3;
    hence thesis by A2,A4,Th1;
  end;
  assume
A5: p in [.s,t.];
  then
A6: s <= p by Th1;
  then
A7: r <= p by A1,XXREAL_0:2;
  p <= t by A5,Th1;
  then
A8: p in [.r,t.] by A7,Th1;
  not p in [.r,s.[ by A6,Th3;
  hence thesis by A8,XBOOLE_0:def 5;
end;
