st: Creating a unique identification number within a group

[Tunga Kantarcı <tungakantarci@gmail.com>]

Hello,

I have three variables (a, b, c) which together uniquely identify a
certain characteristic (call it u). I create a unique identification
number from these three variables using the code

egen u = group(a b c) and this gives:

a b c u
1 2 3 1
1 2 4 2
1 2 4 2
9 2 3 3
9 2 4 4
9 2 4 4

However what I would like to have is that for each "a", "u" starts
from 1 so that I get:

a b c u
1 2 3 1
1 2 4 2
1 2 4 2
9 2 3 1
9 2 4 2
9 2 4 2

Of course in this case u is not a unique identifier in the whole
dataset, but it is so within each a. Could I achieve this?
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