creating and assigning sequence values in pandas dataframe

In the following dataframe, I want to create a sequence number based on the

    0
0   A
1   A
2   A
3   D
4   D
5   A
6   D
7   A
8   D
9   A
10  D

I want to produce

    0   1
0   A   1
1   A   1
2   A   1
3   D   2
4   D   2
5   A   3
6   D   4
7   A   5
8   D   6
9   A   7
10  D   8

solution I tried

diff = d[0].ne(d[0].shift())
d['seqn']=diff.groupby([d[0]]).cumsum()

That is I want to create and assing a value which keeps tracks until it finds new value in the grouupby column. Example if it finds A for the first three rows then they all get value of 1, and then if it finds value of 'D' then it assigns value 2.