Solve the programming task below in a Python markdown code block.
Simply find the closest value to zero from the list. Notice that there are negatives in the list.

List is always not empty and contains only integers. Return ```None``` if it is not possible to define only one of such values. And of course, we are expecting 0 as closest value to zero.

Examples:
```code
[2, 4, -1, -3]  => -1
[5, 2, -2]      => None
[5, 2, 2]       => 2
[13, 0, -6]     => 0
```
Also feel free to reuse/extend the following starter code:
```python
def closest(lst):
```