The expression for $x in E1 return E2 returns the sequence that results from evaluating
E2 once for every item in the sequence E1. Note that E2 must
use the range variable $x to refer to the item being
tested; it does not become the context item. For example, sum(for $v in order-item return
$v/price * $v/quantity) returns the total value of (price times quantity) for all the
selected order-item elements.
