my data looks like this and I cant figure out how to obtain the column "want". I've tried various combinations of retain, lag and sum functions with no success unfortunately.

```
month   quantity1   quantity2   want
1       a           x           x+sum(b to l)
2       b           y           sum(x to y)+sum(c to l)
3       c           z           sum(x to z)+sum(d to l)
4       d       
5       e       
6       f       
7       g       
8       h       
9       i       
10      j       
11      k       
12      l       

```

Thank you for any help on this matter