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

theorem
  p <= q implies [.p,q.] = [.p,q.] \/ [.q,p.]
proof
  assume
A1: p <= q;
  then
A2: [.q,p.] c= {p} by Th85;
  p in [.p,q.] by A1,Th1;
  then {p} c= [.p,q.] by ZFMISC_1:31;
  hence thesis by A2,XBOOLE_1:1,12;
end;
