st: Insert new observation by group

[Fernando Luco <flucoestatalist@gmail.com>]

Dear all,

I'm trying to create a new observation for each group in my data, but
I haven't been able to do so. I tried using -ingap- but didn't work
because
it inserts the new observation at the top of the group or after the
first observation.
My data looks as follows:

group y	 x
1	1	10
1	2	10
1	2	10
2	3	8
2	1	8

What I want is to insert a new observation at the end of each group,
and this observation must be the difference between "x" (in my case
this is 10 for group one and 8 for group two) and the sum of "y"
(6 for group one and 3 for group two). So, the idea is that the final
dataset should look as follows

group y	x
1	1	10
1	2	10
1	2	10
1      5       10
2	3	8
2	1	8
2      4       8

Any ideas? Thanks,

Fernando
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