Thanks,
In the end I separated the code in two parts:
* a part to check all the conditions. (Does node x exist and is it "_") * a part to act on a composition on those conditions. It's a bit longer than before (more documentation) but code is flatter, no redundant branches and much more readable in my opinion. [End code here](https://github.com/mratsim/Arraymancer/blob/580c10b608231a5dcea09620f3cd24bb33a44a12/src/arraymancer/accessors_slicer.nim#L187).
