What if `k` is `'param +'` but `s1` has a key `'param+'` or the other way around?  For example:
```
# a.cfg
[section]
param += 1
```
```
# b.cfg
[buildout]
extends = a.cfg
[section]
# note lack of space in front of `+=`!
param+=2
```
should be handled the same way as
```
# a.cfg
[section]
param += 1
```
```
# b1.cfg
[buildout]
extends = a.cfg
[section]
param += 2
```