diff --git a/cpdfcommand.ml b/cpdfcommand.ml
index 43eac95..62951cd 100644
--- a/cpdfcommand.ml
+++ b/cpdfcommand.ml
@@ -4473,8 +4473,11 @@ let go () =
       let pdf = get_single_pdf args.op true in
       let json = Cpdfua.extract_struct_tree pdf in
         json_to_output json args.out
-  | Some (ReplaceStructTree f) ->
-      ()
+  | Some (ReplaceStructTree s) ->
+      let pdf = get_single_pdf args.op false in
+      let json = Cpdfyojson.Safe.from_file s in
+        Cpdfua.replace_struct_tree pdf json;
+        write_pdf false pdf
 
 (* Advise the user if a combination of command line flags makes little sense,
 or error out if it make no sense at all. *)
diff --git a/cpdfcomposition.ml b/cpdfcomposition.ml
index 7f1235d..6aa2342 100644
--- a/cpdfcomposition.ml
+++ b/cpdfcomposition.ml
@@ -129,13 +129,13 @@ let show_composition_json filesize pdf =
       compressed_xref_table_size pdf
   in
   let r = images + fonts + content_streams + structure_info + attached_files + xref_table in
-    `List [`Tuple [`String "Images"; `Int images; `Float (perc images)];
-           `Tuple [`String "Fonts"; `Int fonts; `Float (perc fonts)];
-           `Tuple [`String "Content streams"; `Int content_streams; `Float (perc content_streams)];
-           `Tuple [`String "Structure Info"; `Int structure_info; `Float (perc structure_info)];
-           `Tuple [`String "Attached Files"; `Int attached_files; `Float (perc attached_files)];
-           `Tuple [`String "XRef Table"; `Int xref_table; `Float (perc xref_table)];
-           `Tuple [`String "Unclassified"; `Int (filesize - r); `Float (perc (filesize - r))]]
+    `List [`List [`String "Images"; `Int images; `Float (perc images)];
+           `List [`String "Fonts"; `Int fonts; `Float (perc fonts)];
+           `List [`String "Content streams"; `Int content_streams; `Float (perc content_streams)];
+           `List [`String "Structure Info"; `Int structure_info; `Float (perc structure_info)];
+           `List [`String "Attached Files"; `Int attached_files; `Float (perc attached_files)];
+           `List [`String "XRef Table"; `Int xref_table; `Float (perc xref_table)];
+           `List [`String "Unclassified"; `Int (filesize - r); `Float (perc (filesize - r))]]
 
 let show_composition_json_blob filesize pdf =
   Pdfio.bytes_of_string (Cpdfyojson.Safe.pretty_to_string (show_composition_json filesize pdf))
@@ -148,7 +148,7 @@ let show_composition filesize json pdf =
     | `List js ->
         iter
           (function
-           | `Tuple [`String a; `Int b; `Float c] -> Printf.printf "%s: %i bytes (%.2f%%)\n" a b c
+           | `List [`String a; `Int b; `Float c] -> Printf.printf "%s: %i bytes (%.2f%%)\n" a b c
            | _ -> ())
         js
     | _ -> ()
diff --git a/cpdfua.ml b/cpdfua.ml
index eadaa57..28f1c94 100644
--- a/cpdfua.ml
+++ b/cpdfua.ml
@@ -339,7 +339,7 @@ let extract_struct_tree pdf =
       | Some x ->
           let objs = Pdf.objects_referenced ["/Pg"; "/Obj"; "/Stm"; "/StmOwn"] [] pdf x in
           let zero =
-            `Tuple [`Int 0;
+            `List [`Int 0;
                     `Assoc [("/CPDFJSONformatversion", `Int 1);
                             ("/CPDFJSONpageobjnumbers", `List (map (fun x -> `Int (unopt (Pdfpage.page_object_number pdf x))) (ilist 1 (Pdfpage.endpage pdf))))]]
           in
@@ -349,7 +349,13 @@ let extract_struct_tree pdf =
                      let jsonobj =
                        Cpdfjson.json_of_object ~utf8:true ~no_stream_data:false ~parse_content:false pdf (function _ -> ()) (Pdf.lookup_obj pdf objnum)
                      in
-                       `Tuple [`Int objnum; jsonobj])
+                       `List [`Int objnum; jsonobj])
                   objs)
       end
   | _ -> error "extract_struct_tree: no root"
+
+(* Use JSON data to replace objects in a file. Negative objects are new ones,
+   we make them positive and renumber them not to clash. Everything else must
+   remain unrenumbered. *)
+let replace_struct_tree pdf json =
+  ()
diff --git a/cpdfua.mli b/cpdfua.mli
index b653cb0..a5c433c 100644
--- a/cpdfua.mli
+++ b/cpdfua.mli
@@ -6,3 +6,5 @@ val test_matterhorn_json : Pdf.t -> Cpdfyojson.Safe.t
 val mark : Pdf.t -> unit
 
 val extract_struct_tree : Pdf.t -> Cpdfyojson.Safe.t
+
+val replace_struct_tree : Pdf.t -> Cpdfyojson.Safe.t -> unit