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

theorem Th321:
  p <= s & p < q implies [.p,q.[ \ ].p,s.[ = {p} \/ [.s,q.[
proof
  [.p,p.] = {p} by Th17;
  hence thesis by Th298;
end;
