Calculating cumulative sum of basal area

Suppose that you have an entity called "tree", a calculated attribute inside of it "tree_num" with the position of the tree (idm:position() expression can be used), and a calculated attribute for the basal area, we can call it ba for example, then you can sum the basal area of the previous inserted trees with an expression like this: sum(parent()/tree[tree_num < $this/parent()/tree_num]/ba). Thanks