diff --git a/src/Pure/Thy/present.ML b/src/Pure/Thy/present.ML
--- a/src/Pure/Thy/present.ML
+++ b/src/Pure/Thy/present.ML
@@ -1,322 +1,332 @@
 (*  Title:      Pure/Thy/present.ML
     Author:     Markus Wenzel and Stefan Berghofer, TU Muenchen
 
 Theory presentation: HTML and PDF-LaTeX documents.
 *)
 
 signature PRESENT =
 sig
   val get_bibtex_entries: theory -> string list
   val theory_qualifier: theory -> string
   val document_option: Options.T -> {enabled: bool, disabled: bool}
   val document_variants: Options.T -> (string * string) list
   val init: HTML.symbols -> bool -> Path.T -> string -> string -> (string * string) list ->
     (Path.T * Path.T) list -> Path.T -> string * string -> bool -> unit
   val finish: unit -> unit
   val theory_output: theory -> string list -> unit
   val begin_theory: string list -> int -> (unit -> HTML.text) -> theory -> theory
 end;
 
 structure Present: PRESENT =
 struct
 
 
 (** paths **)
 
 val tex_ext = Path.ext "tex";
 val tex_path = tex_ext o Path.basic;
 val html_ext = Path.ext "html";
 val html_path = html_ext o Path.basic;
 val index_path = Path.basic "index.html";
 val readme_html_path = Path.basic "README.html";
 val doc_indexN = "session";
 val session_graph_path = Path.basic "session_graph.pdf";
 
 fun show_path path = Path.implode (Path.expand (File.full_path Path.current path));
 
 
 
-(** additional theory data **)
+(** theory data **)
+
+type browser_info = {chapter: string, name: string, bibtex_entries: string list};
+val empty_browser_info: browser_info = {chapter = "Unsorted", name = "Unknown", bibtex_entries = []};
 
 structure Browser_Info = Theory_Data
 (
-  type T = {chapter: string, name: string, bibtex_entries: string list};
-  val empty = {chapter = "Unsorted", name = "Unknown", bibtex_entries = []}: T;
-  fun extend _ = empty;
-  fun merge _ = empty;
+  type T = browser_info
+  val empty = empty_browser_info;
+  val extend = I;
+  val merge = #1;
 );
 
-val _ = Theory.setup
-  (Browser_Info.put {chapter = Context.PureN, name = Context.PureN, bibtex_entries = []});
+fun reset_browser_info thy =
+  if Browser_Info.get thy = empty_browser_info then NONE
+  else SOME (Browser_Info.put empty_browser_info thy);
+
+val _ =
+  Theory.setup
+   (Theory.at_begin reset_browser_info #>
+    Browser_Info.put {chapter = Context.PureN, name = Context.PureN, bibtex_entries = []});
 
 val get_bibtex_entries = #bibtex_entries o Browser_Info.get;
 
 
+
 (** global browser info state **)
 
 (* type theory_info *)
 
 type theory_info = {tex_source: string list, html_source: string};
 
 fun make_theory_info (tex_source, html_source) =
   {tex_source = tex_source, html_source = html_source}: theory_info;
 
 fun map_theory_info f {tex_source, html_source} =
   make_theory_info (f (tex_source, html_source));
 
 
 (* type browser_info *)
 
 type browser_info =
  {theories: theory_info Symtab.table,
   tex_index: (int * string) list,
   html_index: (int * string) list};
 
 fun make_browser_info (theories, tex_index, html_index) : browser_info =
   {theories = theories, tex_index = tex_index, html_index = html_index};
 
 val empty_browser_info = make_browser_info (Symtab.empty, [], []);
 
 fun map_browser_info f {theories, tex_index, html_index} =
   make_browser_info (f (theories, tex_index, html_index));
 
 
 (* state *)
 
 val browser_info = Synchronized.var "browser_info" empty_browser_info;
 fun change_browser_info f = Synchronized.change browser_info (map_browser_info f);
 
 fun init_theory_info name info =
   change_browser_info (fn (theories, tex_index, html_index) =>
     (Symtab.update (name, info) theories, tex_index, html_index));
 
 fun change_theory_info name f =
   change_browser_info (fn (theories, tex_index, html_index) =>
     (case Symtab.lookup theories name of
       NONE => error ("Browser info: cannot access theory document " ^ quote name)
     | SOME info => (Symtab.update (name, map_theory_info f info) theories, tex_index, html_index)));
 
 
 fun add_tex_index txt =
   change_browser_info (fn (theories, tex_index, html_index) =>
     (theories, txt :: tex_index, html_index));
 
 fun add_html_index txt =
   change_browser_info (fn (theories, tex_index, html_index) =>
     (theories, tex_index, txt :: html_index));
 
 
 
 (** global session state **)
 
 (* session_info *)
 
 type session_info =
   {symbols: HTML.symbols, name: string, chapter: string, info_path: Path.T, info: bool,
     doc_format: string, doc_output: Path.T option, doc_files: (Path.T * Path.T) list,
     graph_file: Path.T, documents: (string * string) list, verbose: bool, readme: Path.T option};
 
 fun make_session_info
   (symbols, name, chapter, info_path, info, doc_format, doc_output, doc_files,
     graph_file, documents, verbose, readme) =
   {symbols = symbols, name = name, chapter = chapter, info_path = info_path, info = info,
     doc_format = doc_format, doc_output = doc_output, doc_files = doc_files, graph_file = graph_file,
     documents = documents, verbose = verbose, readme = readme}: session_info;
 
 
 (* state *)
 
 val session_info = Unsynchronized.ref (NONE: session_info option);
 
 fun with_session_info x f = (case ! session_info of NONE => x | SOME info => f info);
 
 val theory_qualifier = Resources.theory_qualifier o Context.theory_long_name;
 
 fun is_session_theory thy =
   (case ! session_info of
     NONE => false
   | SOME {name, ...} => name = theory_qualifier thy);
 
 
 (** document preparation **)
 
 (* options *)
 
 fun document_option options =
   (case Options.string options "document" of
     "" => {enabled = false, disabled = false}
   | "false" => {enabled = false, disabled = true}
   | _ => {enabled = true, disabled = false});
 
 fun document_variants options =
   let
     val variants =
       space_explode ":" (Options.string options "document_variants") |> map (fn s =>
         (case space_explode "=" s of
           [name] => (name, "")
         | [name, tags] => (name, tags)
         | _ => error ("Malformed document variant specification: " ^ quote s)));
     val _ =
       (case duplicates (op =) (map #1 variants) of
         [] => ()
       | dups => error ("Duplicate document variants: " ^ commas_quote dups));
   in variants end;
 
 
 (* init session *)
 
 fun init symbols info info_path doc document_output doc_variants doc_files graph_file
     (chapter, name) verbose =
   let
     val doc_output =
       if document_output = "" then NONE else SOME (Path.explode document_output);
 
     val documents = if doc = "" orelse null doc_files then [] else doc_variants;
     val readme = if File.exists readme_html_path then SOME readme_html_path else NONE;
 
     val docs =
       (case readme of NONE => [] | SOME p => [(Url.File p, "README")]) @
         map (fn (name, _) => (Url.File (Path.ext doc (Path.basic name)), name)) documents;
   in
     session_info :=
       SOME (make_session_info (symbols, name, chapter, info_path, info, doc,
         doc_output, doc_files, graph_file, documents, verbose, readme));
     Synchronized.change browser_info (K empty_browser_info);
     add_html_index (0, HTML.begin_session_index symbols name (Url.File session_graph_path) docs)
   end;
 
 
 (* isabelle tool wrappers *)
 
 fun isabelle_document {verbose} format name tags dir =
   let
     val script =
       "isabelle document -d " ^ File.bash_path dir ^ " -o " ^ Bash.string format ^
         " -n " ^ Bash.string name ^ (if tags = "" then "" else " -t " ^ Bash.string tags);
     val doc_path = Path.appends [dir, Path.parent, Path.basic name |> Path.ext format];
     val _ = if verbose then writeln script else ();
     val {out, err, rc, ...} = Bash.process script;
     val _ = if verbose then writeln (trim_line (normalize_lines out)) else ();
     val _ = if not (File.exists doc_path) orelse rc <> 0 then error (trim_line err) else ();
   in doc_path end;
 
 
 (* finish session -- output all generated text *)
 
 fun sorted_index index = map snd (sort (int_ord o apply2 fst) (rev index));
 fun index_buffer index = Buffer.add (implode (sorted_index index)) Buffer.empty;
 
 fun write_tex src name path =
   File.write_buffer (Path.append path (tex_path name)) src;
 
 fun write_tex_index tex_index path =
   write_tex (index_buffer tex_index) doc_indexN path;
 
 fun finish () =
   with_session_info () (fn {name, chapter, info, info_path, doc_format,
     doc_output, doc_files, graph_file, documents, verbose, readme, ...} =>
   let
     val {theories, tex_index, html_index} = Synchronized.value browser_info;
     val thys = Symtab.dest theories;
 
     val session_prefix =
       Path.append (Path.append info_path (Path.basic chapter)) (Path.basic name);
 
     fun finish_html (a, {html_source, ...}: theory_info) =
       File.write (Path.append session_prefix (html_path a)) html_source;
 
     val _ =
       if info then
        (Isabelle_System.mkdirs session_prefix;
         File.write_buffer (Path.append session_prefix index_path)
           (index_buffer html_index |> Buffer.add HTML.end_document);
         (case readme of NONE => () | SOME path => Isabelle_System.copy_file path session_prefix);
         List.app finish_html thys;
         if verbose
         then Output.physical_stderr ("Browser info at " ^ show_path session_prefix ^ "\n")
         else ())
       else ();
 
     fun document_job doc_prefix backdrop (doc_name, tags) =
       let
         val doc_dir = Path.append doc_prefix (Path.basic doc_name);
         fun purge () = if backdrop then Isabelle_System.rm_tree doc_dir else ();
         val _ = purge ();
         val _ = Isabelle_System.mkdirs doc_dir;
         val _ =
           Isabelle_System.bash ("isabelle latex -o sty " ^
             File.bash_path (Path.append doc_dir (Path.basic "root.tex")));
         val _ = List.app (fn file => Isabelle_System.copy_file_base file doc_dir) doc_files;
         val _ = Isabelle_System.copy_file graph_file (Path.append doc_dir session_graph_path);
         val _ = write_tex_index tex_index doc_dir;
         val _ =
           List.app (fn (a, {tex_source, ...}) =>
             write_tex (fold Buffer.add tex_source Buffer.empty) a doc_dir) thys;
       in
         fn () =>
           (isabelle_document {verbose = true} doc_format doc_name tags doc_dir before purge (),
             fn doc =>
               if verbose orelse not backdrop then
                 Output.physical_stderr ("Document at " ^ show_path doc ^ "\n")
               else ())
       end;
 
     val jobs =
       (if info orelse is_none doc_output then
         map (document_job session_prefix true) documents
        else []) @
       (case doc_output of
         NONE => []
       | SOME path => map (document_job path false) documents);
 
     val _ = jobs |> Par_List.map (fn job => job ()) |> List.app (op |>);
   in
     Synchronized.change browser_info (K empty_browser_info);
     session_info := NONE
   end);
 
 
 (* theory elements *)
 
 fun theory_output thy output =
   with_session_info () (fn _ =>
     if is_session_theory thy then
       (change_theory_info (Context.theory_name thy) o apfst) (K output)
     else ());
 
 fun theory_link (curr_chapter, curr_session) thy =
   let
     val {chapter, name = session, ...} = Browser_Info.get thy;
     val link = html_path (Context.theory_name thy);
   in
     if curr_session = session then SOME link
     else if curr_chapter = chapter then
       SOME (Path.appends [Path.parent, Path.basic session, link])
     else if chapter = Context.PureN then NONE
     else SOME (Path.appends [Path.parent, Path.parent, Path.basic chapter, Path.basic session, link])
   end;
 
 fun begin_theory bibtex_entries update_time mk_text thy =
   with_session_info thy (fn {symbols, name = session_name, chapter, ...} =>
     let
       val name = Context.theory_name thy;
 
       val parent_specs =
         Theory.parents_of thy |> map (fn parent =>
           (Option.map Url.File (theory_link (chapter, session_name) parent),
             (Context.theory_name parent)));
       val html_source = HTML.theory symbols name parent_specs (mk_text ());
       val _ = init_theory_info name (make_theory_info ([], html_source));
 
       val bibtex_entries' =
         if is_session_theory thy then
           (add_html_index (update_time, HTML.theory_entry symbols (Url.File (html_path name), name));
            add_tex_index (update_time, Latex.theory_entry name);
            bibtex_entries)
         else [];
     in
       thy
       |> Browser_Info.put {chapter = chapter, name = session_name, bibtex_entries = bibtex_entries'}
     end);
 
 end;