let p, q, s be ExtReal; :: thesis: ( p <= s & p <= q implies [.p,q.] \ ].p,s.[ = {p} \/ [.s,q.] )
[.p,p.] = {p} by Th17;
hence ( p <= s & p <= q implies [.p,q.] \ ].p,s.[ = {p} \/ [.s,q.] ) by Th300; :: thesis: verum