I've created a pandas DataFrame

```
df=DataFrame(index=['A','B','C'], columns=['x','y'])
```

and got this

x y A NaN NaN B NaN NaN C NaN NaN

Then I want to assign value to particular cell, for example for row 'C' and column 'x'.
I've expected to get such result:

x y A NaN NaN B NaN NaN C 10 NaN

with this code:

```
df.xs('C')['x']=10
```

but contents of **df** haven't changed. It's again only Nan's in dataframe.

Any suggestions?