I have a DataFrame like this:

```
upper    level1    level2 
lower    name      name
1        Mary      Tom
2        ...       ...

```

What should I do if I want to add another column under `level1`? For example

```
upper    level1       level2 
lower    name    age  name
1        Mary    13   Tom
2        ...    ...    ...

```

I can access data with `df['level1'].loc[:,'name']`, but I don't know how to add/remove a column.

If I just use `df.level1['age']=1`, Python returns a copy warning and nothing changed in the DataFrame:

```
SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
if __name__ == '__main__':

```