Suggest a modification of the SkipList structure, such that in addition to the operations insert(x), find(x) and delete(x), you could also answer the operation LesserThan(x) specifying the sum of all the keys in the SkipList are strictly smaller than x.

The expected time for each operation should be O(log n).

Hint: Store at each node v of the SL another field, called size[v], containing the number of keys in the SL between v and the next node at the same level as v. Note that maintaining the values of these fields might imply extra work while performing other operations.