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

theorem
  p <= q implies [.-infty,q.] \ [.p,q.[ = [.-infty,p.[ \/ {q}
proof
  -infty <= q by XXREAL_0:5;
  hence thesis by Th320;
end;
