diff --git a/src/Pure/PIDE/resources.ML b/src/Pure/PIDE/resources.ML
--- a/src/Pure/PIDE/resources.ML
+++ b/src/Pure/PIDE/resources.ML
@@ -1,312 +1,314 @@
 (*  Title:      Pure/PIDE/resources.ML
     Author:     Makarius
 
 Resources for theories and auxiliary files.
 *)
 
 signature RESOURCES =
 sig
   val default_qualifier: string
   val init_session:
     {pide: bool,
      session_positions: (string * Properties.T) list,
      session_directories: (string * string) list,
      docs: string list,
      global_theories: (string * string) list,
      loaded_theories: string list} -> unit
   val finish_session_base: unit -> unit
   val is_pide: unit -> bool
   val global_theory: string -> string option
   val loaded_theory: string -> bool
   val check_session: Proof.context -> string * Position.T -> string
   val check_doc: Proof.context -> string * Position.T -> string
   val master_directory: theory -> Path.T
   val imports_of: theory -> (string * Position.T) list
   val begin_theory: Path.T -> Thy_Header.header -> theory list -> theory
   val thy_path: Path.T -> Path.T
   val theory_qualifier: string -> string
   val find_theory_file: string -> Path.T option
   val import_name: string -> Path.T -> string ->
     {node_name: Path.T, master_dir: Path.T, theory_name: string}
   val check_thy: Path.T -> string ->
    {master: Path.T * SHA1.digest, text: string, theory_pos: Position.T,
     imports: (string * Position.T) list, keywords: Thy_Header.keywords}
   val parse_files: string -> (theory -> Token.file list) parser
   val provide: Path.T * SHA1.digest -> theory -> theory
   val provide_file: Token.file -> theory -> theory
   val provide_parse_files: string -> (theory -> Token.file list * theory) parser
   val loaded_files_current: theory -> bool
   val check_path: Proof.context -> Path.T option -> string * Position.T -> Path.T
   val check_file: Proof.context -> Path.T option -> string * Position.T -> Path.T
   val check_dir: Proof.context -> Path.T option -> string * Position.T -> Path.T
 end;
 
 structure Resources: RESOURCES =
 struct
 
 (* session base *)
 
 val default_qualifier = "Draft";
 
 type entry = {pos: Position.T, serial: serial};
 
 fun make_entry props : entry =
   {pos = Position.of_properties props, serial = serial ()};
 
 val empty_session_base =
   {pide = false,
    session_positions = []: (string * entry) list,
    session_directories = Symtab.empty: Path.T list Symtab.table,
    docs = []: (string * entry) list,
    global_theories = Symtab.empty: string Symtab.table,
    loaded_theories = Symtab.empty: unit Symtab.table};
 
 val global_session_base =
   Synchronized.var "Sessions.base" empty_session_base;
 
 fun init_session
     {pide, session_positions, session_directories, docs, global_theories, loaded_theories} =
   Synchronized.change global_session_base
     (fn _ =>
       {pide = pide,
        session_positions = sort_by #1 (map (apsnd make_entry) session_positions),
        session_directories =
          fold_rev (fn (dir, name) => Symtab.cons_list (name, Path.explode dir))
            session_directories Symtab.empty,
        docs = sort_by #1 (map (apsnd make_entry o rpair []) docs),
        global_theories = Symtab.make global_theories,
        loaded_theories = Symtab.make_set loaded_theories});
 
 fun finish_session_base () =
   Synchronized.change global_session_base
     (fn {global_theories, loaded_theories, ...} =>
       {pide = false,
        session_positions = [],
        session_directories = Symtab.empty,
        docs = [],
        global_theories = global_theories,
        loaded_theories = loaded_theories});
 
 fun get_session_base f = f (Synchronized.value global_session_base);
 
 fun is_pide () = get_session_base #pide;
 
 fun global_theory a = Symtab.lookup (get_session_base #global_theories) a;
 fun loaded_theory a = Symtab.defined (get_session_base #loaded_theories) a;
 
 
 fun check_name which kind markup ctxt arg =
   Completion.check_item kind markup (get_session_base which |> sort_by #1) ctxt arg;
 
 val check_session =
   check_name #session_positions "session"
     (fn (name, {pos, serial}) =>
       Markup.entity Markup.sessionN name
       |> Markup.properties (Position.entity_properties_of false serial pos));
 
 val check_doc = check_name #docs "documentation" (Markup.doc o #1);
 
 
 (* manage source files *)
 
 type files =
  {master_dir: Path.T,  (*master directory of theory source*)
   imports: (string * Position.T) list,  (*source specification of imports*)
   provided: (Path.T * SHA1.digest) list};  (*source path, digest*)
 
 fun make_files (master_dir, imports, provided): files =
  {master_dir = master_dir, imports = imports, provided = provided};
 
 structure Files = Theory_Data
 (
   type T = files;
   val empty = make_files (Path.current, [], []);
-  fun extend _ = empty;
-  fun merge _ = empty;
+  val extend = I;
+  fun merge ({master_dir, imports, provided = provided1}, {provided = provided2, ...}) =
+    let val provided' = Library.merge (op =) (provided1, provided2)
+    in make_files (master_dir, imports, provided') end
 );
 
 fun map_files f =
   Files.map (fn {master_dir, imports, provided} =>
     make_files (f (master_dir, imports, provided)));
 
 
 val master_directory = #master_dir o Files.get;
 val imports_of = #imports o Files.get;
 
 fun begin_theory master_dir {name, imports, keywords} parents =
   Theory.begin_theory name parents
   |> map_files (fn _ => (Path.explode (Path.smart_implode master_dir), imports, []))
   |> Thy_Header.add_keywords keywords;
 
 
 (* theory files *)
 
 val thy_path = Path.ext "thy";
 
 fun theory_qualifier theory =
   (case global_theory theory of
     SOME qualifier => qualifier
   | NONE => Long_Name.qualifier theory);
 
 fun theory_name qualifier theory =
   if Long_Name.is_qualified theory orelse is_some (global_theory theory)
   then theory
   else Long_Name.qualify qualifier theory;
 
 fun find_theory_file thy_name =
   let
     val thy_file = thy_path (Path.basic (Long_Name.base_name thy_name));
     val session = theory_qualifier thy_name;
     val dirs = Symtab.lookup_list (get_session_base #session_directories) session;
   in
     dirs |> get_first (fn dir =>
       let val path = Path.append dir thy_file
       in if File.is_file path then SOME path else NONE end)
   end;
 
 fun make_theory_node node_name theory =
   {node_name = node_name, master_dir = Path.dir node_name, theory_name = theory};
 
 fun loaded_theory_node theory =
   {node_name = Path.basic theory, master_dir = Path.current, theory_name = theory};
 
 fun import_name qualifier dir s =
   let
     val theory = theory_name qualifier (Thy_Header.import_name s);
     fun theory_node () =
       make_theory_node (File.full_path dir (thy_path (Path.expand (Path.explode s)))) theory;
   in
     if not (Thy_Header.is_base_name s) then theory_node ()
     else if loaded_theory theory then loaded_theory_node theory
     else
       (case find_theory_file theory of
         SOME node_name => make_theory_node node_name theory
       | NONE => if Long_Name.is_qualified s then loaded_theory_node theory else theory_node ())
   end;
 
 fun check_file dir file = File.check_file (File.full_path dir file);
 
 fun check_thy dir thy_name =
   let
     val thy_base_name = Long_Name.base_name thy_name;
     val master_file =
       (case find_theory_file thy_name of
         SOME path => check_file Path.current path
       | NONE => check_file dir (thy_path (Path.basic thy_base_name)));
     val text = File.read master_file;
 
     val {name = (name, pos), imports, keywords} =
       Thy_Header.read (Path.position master_file) text;
     val _ =
       thy_base_name <> name andalso
         error ("Bad theory name " ^ quote name ^
           " for file " ^ Path.print (Path.base master_file) ^ Position.here pos);
   in
    {master = (master_file, SHA1.digest text), text = text, theory_pos = pos,
     imports = imports, keywords = keywords}
   end;
 
 
 (* load files *)
 
 fun parse_files cmd =
   Scan.ahead Parse.not_eof -- Parse.path >> (fn (tok, name) => fn thy =>
     (case Token.get_files tok of
       [] =>
         let
           val keywords = Thy_Header.get_keywords thy;
           val master_dir = master_directory thy;
           val pos = Token.pos_of tok;
           val src_paths = Keyword.command_files keywords cmd (Path.explode name);
         in map (Command.read_file master_dir pos) src_paths end
     | files => map Exn.release files));
 
 fun provide (src_path, id) =
   map_files (fn (master_dir, imports, provided) =>
     if AList.defined (op =) provided src_path then
       error ("Duplicate use of source file: " ^ Path.print src_path)
     else (master_dir, imports, (src_path, id) :: provided));
 
 fun provide_file (file: Token.file) = provide (#src_path file, #digest file);
 
 fun provide_parse_files cmd =
   parse_files cmd >> (fn files => fn thy =>
     let
       val fs = files thy;
       val thy' = fold (fn {src_path, digest, ...} => provide (src_path, digest)) fs thy;
     in (fs, thy') end);
 
 fun load_file thy src_path =
   let
     val full_path = check_file (master_directory thy) src_path;
     val text = File.read full_path;
     val id = SHA1.digest text;
   in ((full_path, id), text) end;
 
 fun loaded_files_current thy =
   #provided (Files.get thy) |>
     forall (fn (src_path, id) =>
       (case try (load_file thy) src_path of
         NONE => false
       | SOME ((_, id'), _) => id = id'));
 
 
 (* formal check *)
 
 fun formal_check check_file ctxt opt_dir (name, pos) =
   let
     fun err msg = error (msg ^ Position.here pos);
 
     val _ = Context_Position.report ctxt pos Markup.language_path;
     val dir =
       (case opt_dir of
         SOME dir => dir
       | NONE => master_directory (Proof_Context.theory_of ctxt));
     val path = Path.append dir (Path.explode name) handle ERROR msg => err msg;
     val _ = Path.expand path handle ERROR msg => err msg;
     val _ = Context_Position.report ctxt pos (Markup.path (Path.smart_implode path));
     val _ = check_file path handle ERROR msg => err msg;
   in path end;
 
 val check_path = formal_check I;
 val check_file = formal_check File.check_file;
 val check_dir = formal_check File.check_dir;
 
 
 (* antiquotations *)
 
 local
 
 fun document_antiq (check: Proof.context -> Path.T option -> string * Position.T -> Path.T) =
   Args.context -- Scan.lift (Parse.position Parse.path) >> (fn (ctxt, (name, pos)) =>
     let
       val _ = check ctxt NONE (name, pos);
       val latex = Latex.string (Latex.output_ascii_breakable "/" name);
     in Latex.enclose_block "\\isatt{" "}" [latex] end);
 
 fun ML_antiq check =
   Args.context -- Scan.lift (Parse.position Parse.path) >> (fn (ctxt, (name, pos)) =>
     check ctxt (SOME Path.current) (name, pos) |> ML_Syntax.print_path);
 
 in
 
 val _ = Theory.setup
  (Thy_Output.antiquotation_verbatim_embedded \<^binding>\<open>session\<close>
     (Scan.lift Parse.embedded_position) check_session #>
   Thy_Output.antiquotation_verbatim_embedded \<^binding>\<open>doc\<close>
     (Scan.lift Parse.embedded_position) check_doc #>
   Thy_Output.antiquotation_raw_embedded \<^binding>\<open>path\<close> (document_antiq check_path) (K I) #>
   Thy_Output.antiquotation_raw_embedded \<^binding>\<open>file\<close> (document_antiq check_file) (K I) #>
   Thy_Output.antiquotation_raw_embedded \<^binding>\<open>dir\<close> (document_antiq check_dir) (K I) #>
   ML_Antiquotation.value_embedded \<^binding>\<open>path\<close> (ML_antiq check_path) #>
   ML_Antiquotation.value_embedded \<^binding>\<open>file\<close> (ML_antiq check_file) #>
   ML_Antiquotation.value_embedded \<^binding>\<open>dir\<close> (ML_antiq check_dir) #>
   ML_Antiquotation.value_embedded \<^binding>\<open>path_binding\<close>
     (Scan.lift (Parse.position Parse.path) >>
       (ML_Syntax.print_path_binding o Path.explode_binding)) #>
   ML_Antiquotation.value \<^binding>\<open>master_dir\<close>
     (Args.theory >> (ML_Syntax.print_path o master_directory)));
 
 end;
 
 end;