You can search for identifiers within the package.
in-package search v0.2.0
type id_t = Zmq_deferred__Socket.Make(Zmq_async__.Deferred).Router.id_t
val id_of_string : string -> id_t
val recv : [ `Router ] t -> (id_t * string list) Zmq_async__.Deferred.t
val send : [ `Router ] t -> id_t -> string list -> unit Zmq_async__.Deferred.t