shuffling lines in a stream

[BCS]

Reply to Andrei,

> You don't know the size beforehand, and
> one pass is preferable. The stream can be larger than the working
> memory, but not an unbounded amount of times larger. Also lines in
> the stream can be unbounded, but it is guaranteed that at least a
> nonzero fraction of the file fits in working memory.

I got the bit about the end effect. The above is what I didn't get.