st: Counting interactions within a group

["mai mai" <mai7777@gmail.com>]

Hi,
I have the following data, I want to count the number of times x
interacts with a specific y within each id, I want to create z:
id    x    y   z
s     1    1   1
s     1    3   2
s     1    3   2
s     1    4   1
s     2   1    2
s     2   1    2
s     2   3    1
s     2   4    1
f     1    1    1
f     1    2    2
f    1    2     2
f..........


Many thanks
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