> the comma operator corresponds to union written as | (by definition a
> set is unordered so the additional ordering constraint imposed by the
> , operator is meaningless)

The comma is just a way of separating items in sequence, whereas union
de-dupes and sorts into document order.

select="@foo, @foo, @foo"

vs

select="@foo | @foo | @foo"

the former selects a sequence of 3 items, the latter a sequence of 1.


-- 
Andrew Welch
http://andrewjwelch.com