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

theorem
  for q being Real holds
  [.-infty,q.[ \ ].-infty,s.[ = {-infty} \/ [.s,q.[
proof
  let q be Real;
A1: q in REAL by XREAL_0:def 1;
  -infty <= s by XXREAL_0:5;
  hence thesis by A1,Th321,XXREAL_0:12;
end;
