Not yet, thanks for the tip, I’ll have a look at supporting that too.
Its currently setup to run the expression for each item, in isolation from the other items. I can see why you’d want to do this.
One way to do this is with the Expression element, taking a list of numbers and manipulating the value based on previous (or next) list item …
Map runs the function for each item in the list, and generates a new list.
[Search for Book’s numtype].map(
function(val,pos,list){
return pos > 0 ? list[pos-1] - val : val;
})