As far as I see currently there is no way to access previous rows like lag window function in SQL? https://www.postgresql.org/docs/11/functions-window.html

Still, the function runningAccumulate accesses the values from the previous rows (only within a single Block??).
Looks like it must be relatively easy to implement function runningLag the same way?