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

theorem Th320:
  r <= q & p <= q implies [.r,q.] \ [.p,q.[ = [.r,p.[ \/ {q}
proof
  [.q,q.] = {q} by Th17;
  hence thesis by Th304;
end;
