Fiber.SequenceSourceparallel_iter t ~f is the same as:
let rec loop t ~f = t >>= function | Nil -> return () | Cons (x, t) ->
fork_and_join_unit (fun () -> f x) (fun () -> loop t ~f) except that if the sequence is infinite, the above code would leak memory while parallel_iter does not. This function can typically be used to process a sequence of events.