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

theorem
  for s being Real holds [.-infty,s.[ \ ].-infty,s.[ = {-infty}
proof
  let s be Real;
  s in REAL by XREAL_0:def 1;
  hence thesis by Th404,XXREAL_0:12;
end;
