I have a large dataframe. Here is a small one for the example.

```
    C1  C2  C3  C4
0   foo one 1   4
1   foo one 1   5
2   foo two 2   3
3   bar one 3   6
4   bar two 2   7

```

I perform a list of filters that remove several rows. Here is the final df

```
    C1  C2  C3  C4
0   foo one 1   4
2   foo two 2   3
3   bar one 3   6

```

What I want is the index of the removed lines, so I can output all the values that were rejected.