Recently we had to include a less than statement in a set analysis formula. After some trial and error (and after the help of a colleague) we were able to construct the following statement.
Sum ({$<Year = {$ (vYearCurrent) }, Period = {"<=$(#=$(vPeriodeCurrent) - 1)"}, &#91;Account type&#93; = {'S'} >} [Saldo Actual]) <year (vyearcurrent)="" period="{"<=$(#=$(vPeriodeCurrent)" -="" 1)="" },="" [account="" type]="{'S'}">I hope this helps for anyone who has to overcome the same challenge.