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

theorem  :: LIMFUNC1:18
  for a,b being ExtReal holds ].-infty,b.[ /\ ].a,+infty.[ = ].a,b.[
proof
  let a,b be ExtReal;
A1: -infty <= a by XXREAL_0:5;
  b <= +infty by XXREAL_0:3;
  hence thesis by A1,Th160;
end;
