st: Cumulative Sum by groups

["Alexander Nervedi" <alexnerdy@hotmail.com>]

Hi

I am looking to implement a group level cumulative sum that starts counting 
backwards. Any suggestions on how to go about this would be really useful

data example:
ID  year  num   cusum
1    90      1       2
1    91      1       1
2    90      1       4
2    91      1       3
2    92      1       2
2    93      1       1
3    90      1       1
4    90      1       2
4    90      1       1
...

I have variables ID year and num, and am trying to create cusum

thanks

-AN

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