+<sumof ref="identifier" >expression</sumof>
+
+ The expression is evaluated for each element of the referenced list,
+ in the context of this element.
+ This sumof element then represents a sumation of the results of these
+ evaluations.
+
+ expression will usually be a fieldref which references a field of
+ a list-element or an expression containing a fieldref,
+ such as popcount of a fieldref.
+