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

theorem Th341:
  p <= q implies [.-infty,q.[ \ {p} = [.-infty,p.[ \/ ].p,q.[
proof
  -infty <= p by XXREAL_0:5;
  hence thesis by Th314;
end;
