Sorry, I just realized that the page 159 isn't on google books, so here's the problem in full:
"Exercise12: (a) show that if A is a set of ordinals that contains a largest element alpha then UA is less than or equal to alpha.
(b) give examples of A and alpha as in point (a) such that:
(b1) UA = alpha
(b2) UA is strictly less than alpha"
And it's b2 that I'm having trouble with.
Again, any help would be great.