While Karr's convention in the first place was introduced to attach an interpretation to sums with "wrong" limits,
it can in general be used as a rule telling us what happens if we interchange upper and lower limits.

In integration for example we have:

$\int_a^b f(x) dx   --->   - \int_b^a f(x) dx$ 

see: https://en.wikipedia.org/wiki/Integral#Conventions

Similarly in summation we can apply his rule independently whether $m < n$ or not:

$\sum_{m <= i < n} f(i)  --->  - \sum_{n <= i < m} f(i)$

Applied to a sum we have again:

$\sum_{i=a}^b f(i) = \sum_{a <= i < b+1} f(i) = - \sum_{b+1 <= i < a} f(i) = - \sum_{i=b+1}^{a-1} f(i)$

and hence I suggest the trivial implementation:

```
            if i in indexes:
                e = -e
                l = (limit[0], limit[2] + 1, limit[1] - 1)
            limits.append(l)
```