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

theorem
  p <= s implies [.p,+infty.] \ ].p,s.] = {p} \/ ].s,+infty.]
proof
  p <= +infty by XXREAL_0:3;
  hence thesis by Th324;
end;
