Hi I use it to define my Project WBS (View video)

The script is very similar to each Level, work excelente but and constraint.

let t := this;
let xD := max((select Activities)['Level-1' = t.'Level-1' and 'Level-2' = t.'Level-2'].docID);
if xD = null then docID := 1 else docID := xD + 1 end

There are any way to optimize the formula. 

1. The wbs is in the same table Activities