class type['a]serializer_t =object..end
A serializer queues up engines, and starts the next engine when the previous one finishes.
method serialized : (Unixqueue.event_system -> 'a engine) -> 'a enginelet se = serialized f: Waits until all the previous engines reach
a final state, and then runs e = f esys.
se enters a final state when e does.
