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

theorem
  r <= s implies [.r,+infty.] \ ].p,s.] = [.r,p.] \/ ].s,+infty.]
proof
  p <= +infty by XXREAL_0:3;
  hence thesis by Th308;
end;
