diff --git a/src/pp/containers_pp.ml b/src/pp/containers_pp.ml
index 0b9ea16e..8ca900c0 100644
--- a/src/pp/containers_pp.ml
+++ b/src/pp/containers_pp.ml
@@ -1,4 +1,5 @@
 module B = Buffer
+module Int_map = Map.Make (CCInt)
 
 module Out = struct
   type t = {
@@ -24,10 +25,43 @@ module Out = struct
 end
 
 module Ext = struct
+  type view = ..
+  type 'a key = { id: int; inject: 'a -> view; extract: view -> 'a option }
+  type map = view Int_map.t
+
+  let empty : map = Int_map.empty
+
+  let get k (self : map) : _ option =
+    try k.extract @@ Int_map.find k.id self with Not_found -> None
+
+  let add k v self : map = Int_map.add k.id (k.inject v) self
+
   type 'a t = {
-    pre: Out.t -> 'a -> unit;  (** Printed before the wrapped value. *)
-    post: Out.t -> 'a -> unit;  (** Printed after the wrapped value. *)
+    name: string;
+    k: 'a key;
+    width: 'a -> int;
+    pre: Out.t -> inside:'a option -> 'a -> unit;
+    post: Out.t -> inside:'a option -> 'a -> unit;
   }
+
+  let key_counter_ = ref 0
+
+  let make (type a) ?(width = fun _ -> 0) ~name ~pre ~post () : a t =
+    let module M = struct
+      type view += V of a
+    end in
+    let k =
+      {
+        id = !key_counter_;
+        inject = (fun x -> M.V x);
+        extract =
+          (function
+          | M.V x -> Some x
+          | _ -> None);
+      }
+    in
+    incr key_counter_;
+    { name; k; width; pre; post }
 end
 
 type t = {
@@ -63,7 +97,7 @@ let rec debug out (self : t) : unit =
   | Group d -> Format.fprintf out "(@[group@ %a@])" debug d
   | Fill { sep = _; l } ->
     Format.fprintf out "(@[fill@ %a@])" (Format.pp_print_list debug) l
-  | Wrap (_, _, d) -> Format.fprintf out "(@[ext@ %a@])" debug d
+  | Wrap (e, _, d) -> Format.fprintf out "(@[ext.%s@ %a@])" e.name debug d
 
 let nil : t = { view = Nil; wfl = 0 }
 let newline : t = { view = Newline 1; wfl = 1 }
@@ -98,7 +132,10 @@ let group d : t =
   | Group _ -> d
   | _ -> { view = Group d; wfl = d.wfl }
 
-let ext ext v d : t = { view = Wrap (ext, v, d); wfl = d.wfl }
+let ext (ext : _ Ext.t) v d : t =
+  let wfl = d.wfl + ext.width v in
+  { view = Wrap (ext, v, d); wfl }
+
 let ( ^ ) = append
 let text_sub_ s i len : t = { view = Text_sub (s, i, len); wfl = len }
 
@@ -135,7 +172,7 @@ let textf fmt = Format.kasprintf text fmt
 
 module Flatten = struct
   let to_out (out : Out.t) (self : t) : unit =
-    let rec loop (d : t) =
+    let rec loop (ext_map : Ext.map) (d : t) =
       match d.view with
       | Nil | Newline 0 -> ()
       | Char c -> out.char c
@@ -144,25 +181,27 @@ module Flatten = struct
         for _i = 1 to n do
           out.char ' '
         done
-      | Nest (_, x) -> loop x
+      | Nest (_, x) -> loop ext_map x
       | Append (x, y) ->
-        loop x;
-        loop y
+        loop ext_map x;
+        loop ext_map y
       | Text s | Text_zero_width s -> out.string s
       | Text_sub (s, i, len) -> out.sub_string s i len
-      | Group x -> loop x
+      | Group x -> loop ext_map x
       | Fill { sep; l } ->
         List.iteri
           (fun i x ->
-            if i > 0 then loop sep;
-            loop x)
+            if i > 0 then loop ext_map sep;
+            loop ext_map x)
           l
       | Wrap (ext, v, d) ->
-        ext.pre out v;
-        loop d;
-        ext.post out v
+        let inside = Ext.get ext.k ext_map in
+        ext.pre out ~inside v;
+        let ext_map' = Ext.add ext.k v ext_map in
+        loop ext_map' d;
+        ext.post out ~inside v
     in
-    loop self
+    loop Ext.empty self
 
   let to_buffer buf (self : t) : unit =
     let out = Out.of_buffer buf in
@@ -175,7 +214,7 @@ module Flatten = struct
 end
 
 module Pretty = struct
-  type st = { out: Out.t; width: int }
+  type st = { out: Out.t; width: int; ext_map: Ext.map }
 
   (** Add [i] spaces of indentation. *)
   let add_indent st (i : int) =
@@ -218,9 +257,11 @@ module Pretty = struct
         l;
       !n
     | Wrap (ext, v, d) ->
-      ext.pre st.out v;
-      let n = pp_flatten st d in
-      ext.post st.out v;
+      let inside = Ext.get ext.k st.ext_map in
+      ext.pre st.out ~inside v;
+      let st' = { st with ext_map = Ext.add ext.k v st.ext_map } in
+      let n = pp_flatten st' d in
+      ext.post st.out ~inside v;
       n
 
   (** Does [x] fit in the current line when flattened, given that [k] chars
@@ -238,51 +279,54 @@ module Pretty = struct
     match stack with
     | [] -> ()
     | (i, d) :: stack_tl ->
-      pp_rec_top st ~k ~i d (fun k -> pp_rec st k stack_tl)
+      pp_rec_top st ~k ~i d (fun st k -> pp_rec st k stack_tl)
 
   (** Print [d] at indentation [i], with [k] chars already printed
       on the current line, then calls [kont] with the
       new [k]. *)
-  and pp_rec_top st ~k ~i d (kont : int -> unit) : unit =
+  and pp_rec_top st ~k ~i d (kont : st -> int -> unit) : unit =
     match d.view with
-    | Nil -> kont k
+    | Nil -> kont st k
     | Char c ->
       st.out.char c;
-      kont (k + 1)
+      kont st (k + 1)
     | Newline _ ->
       pp_newline st i;
-      kont i
+      kont st i
     | Nest (j, x) -> pp_rec_top st ~k ~i:(i + j) x kont
     | Append (x, y) ->
       (* print [x], then print [y] *)
-      pp_rec_top st ~k ~i x (fun k -> pp_rec_top st ~k ~i y kont)
+      pp_rec_top st ~k ~i x (fun st k -> pp_rec_top st ~k ~i y kont)
     | Text s ->
       st.out.string s;
-      kont (k + String.length s)
+      kont st (k + String.length s)
     | Text_zero_width s ->
       st.out.string s;
-      kont k
+      kont st k
     | Text_sub (s, i, len) ->
       st.out.sub_string s i len;
-      kont (k + len)
+      kont st (k + len)
     | Group x ->
       if fits_flattened st k x then (
         (* print flattened *)
         let w_x = pp_flatten st x in
         assert (w_x = x.wfl);
-        kont (k + w_x)
+        kont st (k + w_x)
       ) else
         pp_rec_top st ~k ~i x kont
     | Fill { sep; l } -> pp_fill st ~k ~i sep l kont
     | Wrap (ext, v, d) ->
-      ext.pre st.out v;
-      pp_rec_top st ~k ~i d (fun k ->
-          ext.post st.out v;
-          kont k)
+      let old_ext_map = st.ext_map in
+      let inside = Ext.get ext.k st.ext_map in
+      ext.pre st.out ~inside v;
+      let st' = { st with ext_map = Ext.add ext.k v st.ext_map } in
+      pp_rec_top st' ~k ~i d (fun st k ->
+          ext.post st.out ~inside v;
+          kont { st with ext_map = old_ext_map } k)
 
-  and pp_fill st ~k ~i sep l (kont : int -> unit) : unit =
+  and pp_fill st ~k ~i sep l (kont : st -> int -> unit) : unit =
     (* [k] is the current offset in the line *)
-    let rec loop idx k l =
+    let rec loop st idx k l =
       match l with
       | x :: tl ->
         if fits_flattened st k x then (
@@ -295,24 +339,24 @@ module Pretty = struct
           in
           let w_x = pp_flatten st x in
           assert (w_x = x.wfl);
-          loop (idx + 1) (k + w_x + w_sep) tl
+          loop st (idx + 1) (k + w_x + w_sep) tl
         ) else (
           (* print, followed by a newline and resume filling with [k=i] *)
-          let pp_and_continue k =
-            pp_rec_top st ~k ~i x (fun k -> loop (idx + 1) k tl)
+          let pp_and_continue st k =
+            pp_rec_top st ~k ~i x (fun st k -> loop st (idx + 1) k tl)
           in
           if idx > 0 then
             (* separator, then item *)
             pp_rec_top st ~k ~i sep pp_and_continue
           else
-            pp_and_continue k
+            pp_and_continue st k
         )
-      | [] -> kont k
+      | [] -> kont st k
     in
-    loop 0 k l
+    loop st 0 k l
 
   let to_out ~width out (self : t) : unit =
-    let st = { out; width } in
+    let st = { out; width; ext_map = Ext.empty } in
     pp_rec st 0 [ 0, self ]
 
   let to_buffer ~width (buf : Buffer.t) (self : t) : unit =
@@ -464,12 +508,15 @@ module Term_color = struct
       Buffer.add_string buf "m";
       Buffer.contents buf
 
-  (* TODO: handle nested styles *)
   let ext_style_ : style list Ext.t =
-    {
-      pre = (fun out l -> Out.string out (string_of_style_list l));
-      post = (fun out _l -> Out.string out reset);
-    }
+    Ext.make ~name:"termcolor"
+      ~pre:(fun out ~inside:_ l -> Out.string out (string_of_style_list l))
+      ~post:(fun out ~inside _l ->
+        let style =
+          CCOption.map_or ~default:reset string_of_style_list inside
+        in
+        Out.string out style)
+      ()
 
   (** Set the foreground color. *)
   let color (c : color) (d : t) : t = ext ext_style_ [ `FG c ] d
diff --git a/src/pp/containers_pp.mli b/src/pp/containers_pp.mli
index fe502348..80cce5ae 100644
--- a/src/pp/containers_pp.mli
+++ b/src/pp/containers_pp.mli
@@ -120,15 +120,26 @@ end
     might be annotated with ANSI-terminal colors, or
     with HTML tags. *)
 module Ext : sig
-  type 'a t = {
-    pre: Out.t -> 'a -> unit;  (** Printed before the wrapped value. *)
-    post: Out.t -> 'a -> unit;  (** Printed after the wrapped value. *)
-  }
+  type 'a t
+
+  val make :
+    ?width:('a -> int) ->
+    name:string ->
+    pre:(Out.t -> inside:'a option -> 'a -> unit) ->
+    post:(Out.t -> inside:'a option -> 'a -> unit) ->
+    unit ->
+    'a t
   (**  An extension is a custom document node. It takes a value of type ['a],
       and a document [d], and can output what it wants based
       on the custom value before and after [d] is printed.
 
-      The extension is considered to have width [0]. *)
+      The extension is considered to have width [0], unless [width]
+      is specified.
+
+      @param pre called before the wrapped value is printed
+      @param post called after the wrapped value is printed
+
+      *)
 end
 
 val ext : 'a Ext.t -> 'a -> t -> t