theorem
  [.-infty,+infty.] = ExtREAL
proof
  let r;
  thus r in [.-infty,+infty.] implies r in ExtREAL by XXREAL_0:def 1;
  assume r in ExtREAL;
A1: -infty <= r by XXREAL_0:5;
  r <= +infty by XXREAL_0:3;
  hence thesis by A1,Th1;
end;
