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

theorem Th317:
  r < q & p <= q implies ].r,q.] \ ].p,q.[ = ].r,p.] \/ {q}
proof
  [.q,q.] = {q} by Th17;
  hence thesis by Th299;
end;
