Subject Howto iterate through all selected rows
How do I iterate through all selected rows in a query, that is have a
loop that gets executed for every row where the selected property is

I know about the selected[] array, but it is pretty ineffective to
iterate through *all* rows, just to find those (few) that are
actually selected.

SelectedBookmarks looks good, but what if the user has selected a
very large number of rows (e.g 100.000 via SelectAll)?
SelectedBookmarks will return a huge list, taking up more memory than
the computer might have...