Any location can be augmented by a binding of user-defined variables to expressions. The expression values are available when processing the descendants of the assigning node. In the following example,

   <a:Images 
      vars="group=string(. idiv 50 * 50) ; folder=concat('f', $group)"
      ...> ...
   </a:Images>

two variables are introduced, group and folder, whose values are available when constructing the contents of an <a:Images> element:

   <a:Section src="$folder"/>