0 - 2 of 2 tags for cumulative sum

Hi All, 
I'm trying to come up with an OLAP function solution to a units deduction problem for various groups. 

Hi - I've seen some posts here regarding the SUM OVER function to calculate the cumulative sum.  My problem at hand is slightly different, where I need to keep a running total of distinct purchasers by year:
Year        #Distinct Customers         #Cumulative (Distinct Customers)