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

theorem
  [.r,+infty.] \ ].p,+infty.[ = [.r,p.] \/ {+infty}
proof
A1: r <= +infty by XXREAL_0:3;
  p <= +infty by XXREAL_0:3;
  hence thesis by A1,Th318;
end;
