st: Creating missing values

["MA V" <ma_v@hotmail.com>]

Hi,
My data is the following:

obs v1  v2
1     1  20
2     1  34
3     1  34
4    1  34
5     1  32
2     2  23
4     2  32
5     2  23
And I would like to make it look like:
obs v1  v2
1     1  20
2     1  34
3     1  34
4     1  34
5     1  32
1     2  .
2     2  23
3     2  .
4     2  32
5     2  23
How do I add the missing values? Note that I want to have the same number of 
observations for each v1.
Can you help me?
MA

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