st: Computing a cumulative frequency variable

["Raphael Fraser" <raphael.fraser@gmail.com>]

I have the following variables and would like to generate or compute
the cumulative frequency of etime. The end result should look like
this:

id    etime  cum. freq
15     0       0
15    6        6
15    2        8
20    10      10
20    30       40
20    4         44

Raphael

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