Maybe, but if you can segment out the unobserved items, and back-calculate it lazily, you could save a lot of processing and memory. The place to implement an

Maybe, but if you can segment out the unobserved items, and back-calculate it lazily, you could save a lot of processing and memory. The place to implement an optimization like this would be at the items with the biggest O in your world, which is usually the smallest building block--what you'd have the "most" of, which would drive the largest memory and processing demands.