diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index eba351b..65ff292 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -32,6 +32,11 @@ jobs:
           dune-cache: true
           allow-prerelease-opam: true
 
+      # check that trace compiles with no optional deps
+      - run: opam install -t trace --deps-only
+      - run: opam exec -- dune build '@install' -p trace
+
+      # install all packages
       - run: opam install -t trace trace-tef trace-fuchsia --deps-only
       - run: opam install ppx_trace --deps-only # no tests
         if: matrix.ocaml-compiler != '4.08.x'
diff --git a/README.md b/README.md
index b10a258..99e943b 100644
--- a/README.md
+++ b/README.md
@@ -150,9 +150,18 @@ Concrete tracing or observability formats such as:
   * [x] backend for [tldrs](https://github.com/imandra-ai/tldrs), a
         small rust daemon that aggregates TEF traces from multiple processes/clients
         into a single `.jsonl` file
-  * [ ] richer bindings with [ocaml-catapult](https://github.com/imandra-ai/catapult),
-        with multi-process backends, etc.
+  * [x] [tldrs](https://github.com/imandra-ai/tldrs), to collect TEF traces from multiple processes in a clean way.
+        This requires the rust `tldrs` program to be in path.
+  * ~~[ ] richer bindings with [ocaml-catapult](https://github.com/imandra-ai/catapult),
+        with multi-process backends, etc.~~ (subsumed by tldrs)
 - [x] Tracy (see [ocaml-tracy](https://github.com/imandra-ai/ocaml-tracy), more specifically `tracy-client.trace`)
 - [x] Opentelemetry (see [ocaml-opentelemetry](https://github.com/imandra-ai/ocaml-opentelemetry/), in `opentelemetry.trace`)
 - [ ] landmarks?
 - [ ] Logs (only for messages, obviously)
+
+## Subscribers
+
+The library `trace.subscriber` defines composable _subscribers_, which are sets of callbacks
+that consume tracing events.
+Multiple subscribers can be aggregated together (with events being dispatched to all of them)
+and be installed as a normal _collector_.
diff --git a/src/subscriber/callbacks.ml b/src/subscriber/callbacks.ml
index 3ff7647..112bf51 100644
--- a/src/subscriber/callbacks.ml
+++ b/src/subscriber/callbacks.ml
@@ -1,3 +1,23 @@
+(** Callbacks used for subscribers.
+
+  Each subscriber defines a set of callbacks, for each possible
+  tracing event. These callbacks take a custom state that is paired
+  with the callbacks in {!Subscriber.t}.
+
+  To use a default implementation for some callbacks, use:
+
+  {[
+  module My_callbacks = struct
+    type st = …
+
+    include Trace_subscriber.Callbacks.Dummy
+
+    let on_init (state:st) ~time_ns : unit = …
+
+    (* other customize callbacks *)
+  end ]}
+*)
+
 open Trace_core
 open Types
 
@@ -119,7 +139,7 @@ module Dummy = struct
     ()
 end
 
-(** Dummy callbacks, do nothing. *)
+(** Dummy callbacks, ignores all events. *)
 let dummy (type st) () : st t =
   let module M = struct
     type nonrec st = st
diff --git a/src/subscriber/subscriber.ml b/src/subscriber/subscriber.ml
index 0246984..46c100c 100644
--- a/src/subscriber/subscriber.ml
+++ b/src/subscriber/subscriber.ml
@@ -1,9 +1,11 @@
+(** Trace subscribers *)
+
 (** A trace subscriber. It pairs a set of callbacks
     with the state they need (which can contain a file handle,
-    a socket, config, etc.).
+    a socket to write events to, config, etc.).
 
     The design goal for this is that it should be possible to avoid allocations
-    when the trace collector calls the callbacks. *)
+    whenever the trace collector invokes the callbacks. *)
 type t =
   | Sub : {
       st: 'st;
@@ -102,3 +104,12 @@ end
 let tee (s1 : t) (s2 : t) : t =
   let st = s1, s2 in
   Sub { st; callbacks = (module Tee_cb) }
+
+(** Tee multiple subscribers, ie return a subscriber that forwards
+    to all the subscribers in [subs]. *)
+let rec tee_l (subs : t list) : t =
+  match subs with
+  | [] -> dummy
+  | [ s ] -> s
+  | [ s1; s2 ] -> tee s1 s2
+  | s1 :: s2 :: tl -> tee (tee s1 s2) (tee_l tl)
diff --git a/src/subscriber/trace_subscriber.ml b/src/subscriber/trace_subscriber.ml
index bbe4626..fe13910 100644
--- a/src/subscriber/trace_subscriber.ml
+++ b/src/subscriber/trace_subscriber.ml
@@ -13,9 +13,7 @@ module Private_ = struct
   let[@inline] now_ns () : float =
     match !get_now_ns_ with
     | Some f -> f ()
-    | None ->
-      let t = Mtime_clock.now () in
-      Int64.to_float (Mtime.to_uint64_ns t)
+    | None -> Time_.get_time_ns ()
 
   let[@inline] tid_ () : int =
     match !get_tid_ with
diff --git a/src/subscriber/trace_subscriber.mli b/src/subscriber/trace_subscriber.mli
index bba7982..a20daf0 100644
--- a/src/subscriber/trace_subscriber.mli
+++ b/src/subscriber/trace_subscriber.mli
@@ -15,13 +15,17 @@ include module type of struct
   include Types
 end
 
+(** {2 Main API} *)
+
 type t = Subscriber.t
 
 val collector : t -> Trace_core.collector
+(** A collector that calls the subscriber's callbacks.
+
+  It uses [mtime] (if available) to obtain timestamps. *)
 
 (**/**)
 
-(**/*)
 module Private_ : sig
   val get_now_ns_ : (unit -> float) option ref
   (** The callback used to get the current timestamp *)
@@ -31,3 +35,5 @@ module Private_ : sig
 
   val now_ns : unit -> float
 end
+
+(**/**)
diff --git a/src/subscriber/types.ml b/src/subscriber/types.ml
index e507f46..dc48841 100644
--- a/src/subscriber/types.ml
+++ b/src/subscriber/types.ml
@@ -1,10 +1,13 @@
+(** Some core types for subscribers. *)
+
 type user_data =
   | U_bool of bool
   | U_float of float
   | U_int of int
   | U_none
   | U_string of string
+      (** A non polymorphic-variant version of {!Trace_core.user_data} *)
 
 type flavor =
   | Sync
-  | Async
+  | Async  (** A non polymorphic-variant version of {!Trace_core.flavor} *)