@jling Yeah it’s a poor minimal example, but it does work - the error you’re seeing is due to > having no defined identity element. If you provide an init=0, for example, it will work.
The actual use case here is a type of 2d fourier transform. It’s a bit too heavy to get into the details, but the map operator is a little exponential, and the reduction sums across all the data points onto a 2D grid.