Hi Sebastian,
That looks nice, I don't have a clear feeling on the order of items, if we think of it in terms of `(start, stop)` there was also the idea voiced to simply add another name in which case you would allow start and stop to be separate arrays.
Yes, one could add another method. Or perhaps even add a new argument to `.reduce` instead (say `slices`). But this seemed the simplest route...
Of course if go with your `slice(start, stop)` idea that also works, although passing as separate parameters seems nice too.
Adding another name (if we can think of one at least) seems pretty good to me, since I suspect we would add docs to suggest not using `reduceat`.
If we'd want to, even with the present PR it would be possible to (very slowly) deprecate the use of a list of single integers. But I'm trying to go with just making the existing method more useful.
One small thing about the PR: I would like to distinct `default` and `initial`. I.e. the default value is used only for empty reductions, while the initial value should be always used (unless you would pass both, which we don't for normal reductions though). I suppose the machinery isn't quite set up to do both side-by-side.
I just followed what is done for reduce, where a default could also have made sense given that `where` can exclude all inputs along a given row. I'm not convinced it would be necessary to have both, though it would not be hard to add. All the best, Marten