I'm suprised that there's no
equivalent of the @exf:iterate attribute
(http://www.exforms.org/conditionals.html#id4484032) in XForms 1.1, to
go with the @while attribute.
You can achieve the same thing with @while, but it's very cumbersome
because the context node doesn't change, so you need to initialise a
separate counter & manually increment it.