Wowthis generated much more discussion than I anticipated.

For my requirements of the moment the order of the result node set is
unimportant, so $nodes intersect () seems like the least surprising solution
(and I suspect I might have arrived at it if Id spent a bit more time
thinking about the problem, but its the holidays and I had things to do and
places to be ).

Ive never actually taken the time to understand how fold-left works, so this
is an opportunity to learn.