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

theorem :: LIMFUNC1:19
  for a,b being Real holds ].-infty,a.] /\ [.b,+infty.[ = [.b,a.]
proof
  let a,b be Real;
A1: a in REAL by XREAL_0:def 1;
  b in REAL by XREAL_0:def 1;
  then
A2: -infty < b by XXREAL_0:12;
  a < +infty by A1,XXREAL_0:9;
  hence thesis by A2,Th152;
end;
