st: how to -expand- and attach new group id to panel data

["sdm1" <sdm1@york.ac.uk>]

I have a balanced panel data set with 3 annual observations for each of 3
groups e.g.,

Group_id	year
1		78
1		79
1		80
2		78
2		79
2		80
3		78
3		79
3		80

Suppose I use -expand- to generate, say, one extra copy of the data for
group_id 2 and two extra copies of the data for group_id 3.  Now I have:

Group_id	year
1		78
1		79
1		80
2		78
2		79
2		80
3		78
3		79
3		80
2		78
2		79
2		80
3		78
3		79
3		80 
3		78
3		79
3		80  

How do I give new group_ids to the copied data so that now: 

Group_id	year	Newgroup_id
1		78	1
1		79	1
1		80	1
2		78	2
2		79	2
2		80	2
3		78	3
3		79	3
3		80	3
2		78	4
2		79	4
2		80	4
3		78	5
3		79	5
3		80 	5
3		78	6
3		79	6
3		80  	6

Thanks.

Steve

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