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 p <= q
  implies ].-infty,q.] \ [.p,q.[ = ].-infty,p.[ \/ {q}
proof
  let q be Real;
  q in REAL by XREAL_0:def 1;
  hence thesis by Th319,XXREAL_0:12;
end;
