Partial sum implementation

Continuation indent means that when you have a single line:
```c++
auto long_name = tooo_looong_expr; 
```

and that gets split like this:

```c++
auto long_name 
__ = tooo_looong_expr; 
//^ this is the indentantion for when a statement needs to be broken because
//  it is too long;
```
I can set it to 4. Or whatever should be used here. I honestly don't really care as long as it is consistent and one can identify that the thing is a statement that continues on the next line.