I have a `pandas.DataFrame`:

```
    index    question_id    tag
    0        1858           [pset3, game-of-fifteen]
    1        2409           [pset4]
    2        4346           [pset6, cs50submit]
    3        9139           [pset8, pset5, gradebook]
    4        9631           [pset4, recover]

```

I need to remove every string from list of strings in `tag` column except `pset*` strings. 

So I need to end with something like this:

```
    index    question_id    tag
    0        1858           [pset3]
    1        2409           [pset4]
    2        4346           [pset6]
    3        9139           [pset8, pset5]
    4        9631           [pset4]

```

How can I do that please?