Thank you jerf and sampo for your very helpful comments. This is something that I had on my mind during the implementation but focused primarily on expressivene

Thank you jerf and sampo for your very helpful comments. This is something that I had on my mind during the implementation but focused primarily on expressiveness. While the array slicing in Fortran is O(1), the implementation of tail does make a copy. Though a good compiler should be able to replace tail(x) with x(2:), the Fortran standard does not guarantee it to do so. The first step forward that I see would be to replace tail(x) with x(2:) in the internal implementations of fold[lr]. In any case, the time complexity for these functions is definitely something I plan to document soon.