module Sequencer:A sequencer is a throttle that is:sig..end
    1. specialized to only allow one job at a time and to not continue on error, and
    2. generalized to carry its own state, and enforce mutually exclusive access to that
      state by the jobs
type 'a t 
val create : ?continue_on_error:bool -> 'a -> 'a tval enqueue : 'a t -> ('a -> 'b Deferred.t) -> 'b Deferred.tval num_jobs_waiting_to_start : 'a t -> int