toplevel emits round trip pprint of ast to console.

toplevel.ml

@@ -28,9 +28,25 @@ module Graphics_support = struct
   let init elt = Graphics_js.open_canvas elt
 end
 
+open Log_js
+
 module Ppx_support = struct
   let init () =
-    Ast_mapper.register "js_of_ocaml" (fun _ -> Ppx_js.mapper)
+    Ppx_graph.init ();
+    Ast_mapper.register "js_of_ocaml" (fun _ -> Ppx_js.mapper);
+    Ast_mapper.register "ppxlib" (fun _ ->
+        Log.info (fun m -> m "Ppxlib.mapper");
+        {
+          Ast_mapper.default_mapper with
+          structure =
+            (fun _ ->
+              Log.info (fun m -> m "Ppxlib.Driver.map_structure");
+              Ppxlib.Driver.map_structure);
+          signature =
+            (fun _ ->
+              Log.info (fun m -> m "Ppxlib.Driver.map_signature");
+              Ppxlib.Driver.map_signature);
+        })
 end
 
 module Colorize = struct