Cumulative frequency of unique values within groups

Hello statalist,

My dataset is an unbalanced time series (with repeated years). Each company can have several classifications per year, ranging from 1 to * (i.e., there can be hundreds). Below is a snapshot of the dataset for 1 company with classifications A, B, C

company	year	classification
1	1985	A
1	1991	A
1	1993	B
1	1993	B
1	1993	B
1	1994	A
1	1994	A
1	1994	B
1	1994	B
1	1994	B
1	1994	C
1	1994	C
1	1994	C
1	1994	C
1	1994	C

I would like to create cumulative frequency for the classifications by company and year, however, I would like the count to only sum the same values of classification.

I started by creating a frequency var:
. bysort company year classification: gen freq = _N
Also created a tag:
. egen tag = tag(company year classification)

company	Year	classification	freq	tag
1	1985	A	1	1
1	1991	A	1	1
1	1993	B	3	1
1	1993	B	3	0
1	1993	B	3	0
1	1994	A	2	1
1	1994	A	2	0
1	1994	B	3	1
1	1994	B	3	0
1	1994	B	3	0
1	1994	C	5	1
1	1994	C	5	0
1	1994	C	5	0
1	1994	C	5	0
1	1994	C	5	0

Now, summing the freq var by(company year classification) with egen does not produce the cumulative frequency I'm looking for. I would like the cumulative count to refer to the same type of class only. If I call this var 'cum', then this is what I am looking for:

company	Year	classification	freq	tag	cum
1	1985	A	1	1	1
1	1991	A	1	1	2
1	1993	B	3	1	3
1	1993	B	3	0	3
1	1993	B	3	0	3
1	1994	A	2	1	4
1	1994	A	2	0	4
1	1994	B	3	1	6
1	1994	B	3	0	6
1	1994	B	3	0	6
1	1994	C	5	1	5
1	1994	C	5	0	5
1	1994	C	5	0	5
1	1994	C	5	0	5
1	1994	C	5	0	5

How can I create such cumulative var?

Thank you.