let p, q be ExtReal; :: thesis: ( p <= q implies [.-infty,q.] \ [.p,q.[ = [.-infty,p.[ \/ {q} )
-infty <= q by XXREAL_0:5;
hence ( p <= q implies [.-infty,q.] \ [.p,q.[ = [.-infty,p.[ \/ {q} ) by Th320; :: thesis: verum