diff --git a/src/Pure/PIDE/document.scala b/src/Pure/PIDE/document.scala
--- a/src/Pure/PIDE/document.scala
+++ b/src/Pure/PIDE/document.scala
@@ -1,1198 +1,1199 @@
 /*  Title:      Pure/PIDE/document.scala
     Author:     Makarius
 
 Document as collection of named nodes, each consisting of an editable
 list of commands, associated with asynchronous execution process.
 */
 
 package isabelle
 
 
 import scala.collection.mutable
 
 
 object Document
 {
   /** document structure **/
 
   /* overlays -- print functions with arguments */
 
   object Overlays
   {
     val empty = new Overlays(Map.empty)
   }
 
   final class Overlays private(rep: Map[Node.Name, Node.Overlays])
   {
     def apply(name: Node.Name): Node.Overlays =
       rep.getOrElse(name, Node.Overlays.empty)
 
     private def update(name: Node.Name, f: Node.Overlays => Node.Overlays): Overlays =
     {
       val node_overlays = f(apply(name))
       new Overlays(if (node_overlays.is_empty) rep - name else rep + (name -> node_overlays))
     }
 
     def insert(command: Command, fn: String, args: List[String]): Overlays =
       update(command.node_name, _.insert(command, fn, args))
 
     def remove(command: Command, fn: String, args: List[String]): Overlays =
       update(command.node_name, _.remove(command, fn, args))
 
     override def toString: String = rep.mkString("Overlays(", ",", ")")
   }
 
 
   /* document blobs: auxiliary files */
 
   sealed case class Blob(bytes: Bytes, source: String, chunk: Symbol.Text_Chunk, changed: Boolean)
   {
     def unchanged: Blob = copy(changed = false)
   }
 
   object Blobs
   {
     def apply(blobs: Map[Node.Name, Blob]): Blobs = new Blobs(blobs)
     val empty: Blobs = apply(Map.empty)
   }
 
   final class Blobs private(blobs: Map[Node.Name, Blob])
   {
     def get(name: Node.Name): Option[Blob] = blobs.get(name)
 
     def changed(name: Node.Name): Boolean =
       get(name) match {
         case Some(blob) => blob.changed
         case None => false
       }
 
     override def toString: String = blobs.mkString("Blobs(", ",", ")")
   }
 
 
   /* document nodes: theories and auxiliary files */
 
   type Edit[A, B] = (Node.Name, Node.Edit[A, B])
   type Edit_Text = Edit[Text.Edit, Text.Perspective]
   type Edit_Command = Edit[Command.Edit, Command.Perspective]
 
   object Node
   {
     /* header and name */
 
     sealed case class Header(
       imports_pos: List[(Name, Position.T)] = Nil,
       keywords: Thy_Header.Keywords = Nil,
       abbrevs: Thy_Header.Abbrevs = Nil,
       errors: List[String] = Nil)
     {
       def imports: List[Name] = imports_pos.map(_._1)
 
       def append_errors(msgs: List[String]): Header =
         copy(errors = errors ::: msgs)
 
       def cat_errors(msg2: String): Header =
         copy(errors = errors.map(msg1 => Exn.cat_message(msg1, msg2)))
     }
 
     val no_header = Header()
     def bad_header(msg: String): Header = Header(errors = List(msg))
 
     object Name
     {
       val empty = Name("")
 
       object Ordering extends scala.math.Ordering[Name]
       {
         def compare(name1: Name, name2: Name): Int = name1.node compare name2.node
       }
 
       type Graph[A] = isabelle.Graph[Node.Name, A]
 
       def make_graph[A](entries: List[((Name, A), List[Name])]): Graph[A] =
         Graph.make(entries, symmetric = true)(Ordering)
     }
 
     sealed case class Name(node: String, master_dir: String = "", theory: String = "")
     {
       override def hashCode: Int = node.hashCode
       override def equals(that: Any): Boolean =
         that match {
           case other: Name => node == other.node
           case _ => false
         }
 
       def path: Path = Path.explode(File.standard_path(node))
       def master_dir_path: Path = Path.explode(File.standard_path(master_dir))
 
       def is_theory: Boolean = theory.nonEmpty
 
       def theory_base_name: String = Long_Name.base_name(theory)
 
       override def toString: String = if (is_theory) theory else node
 
       def map(f: String => String): Name = copy(f(node), f(master_dir), theory)
       def map_theory(f: String => String): Name = copy(node, master_dir, f(theory))
 
       def json: JSON.Object.T =
         JSON.Object("node_name" -> node, "theory_name" -> theory)
     }
 
     sealed case class Entry(name: Node.Name, header: Node.Header)
     {
       def map(f: String => String): Entry = copy(name = name.map(f))
 
       override def toString: String = name.toString
     }
 
 
     /* node overlays */
 
     object Overlays
     {
       val empty = new Overlays(Multi_Map.empty)
     }
 
     final class Overlays private(rep: Multi_Map[Command, (String, List[String])])
     {
       def commands: Set[Command] = rep.keySet
       def is_empty: Boolean = rep.isEmpty
       def dest: List[(Command, (String, List[String]))] = rep.iterator.toList
       def insert(cmd: Command, fn: String, args: List[String]): Overlays =
         new Overlays(rep.insert(cmd, (fn, args)))
       def remove(cmd: Command, fn: String, args: List[String]): Overlays =
         new Overlays(rep.remove(cmd, (fn, args)))
 
       override def toString: String = rep.mkString("Node.Overlays(", ",", ")")
     }
 
 
     /* edits */
 
     sealed abstract class Edit[A, B]
     {
       def foreach(f: A => Unit)
       {
         this match {
           case Edits(es) => es.foreach(f)
           case _ =>
         }
       }
 
       def is_void: Boolean =
         this match {
           case Edits(Nil) => true
           case _ => false
         }
     }
     case class Blob[A, B](blob: Document.Blob) extends Edit[A, B]
 
     case class Edits[A, B](edits: List[A]) extends Edit[A, B]
     case class Deps[A, B](header: Header) extends Edit[A, B]
     case class Perspective[A, B](required: Boolean, visible: B, overlays: Overlays) extends Edit[A, B]
 
 
     /* perspective */
 
     type Perspective_Text = Perspective[Text.Edit, Text.Perspective]
     type Perspective_Command = Perspective[Command.Edit, Command.Perspective]
 
     val no_perspective_text: Perspective_Text =
       Perspective(false, Text.Perspective.empty, Overlays.empty)
 
     val no_perspective_command: Perspective_Command =
       Perspective(false, Command.Perspective.empty, Overlays.empty)
 
     def is_no_perspective_text(perspective: Perspective_Text): Boolean =
       !perspective.required &&
       perspective.visible.is_empty &&
       perspective.overlays.is_empty
 
     def is_no_perspective_command(perspective: Perspective_Command): Boolean =
       !perspective.required &&
       perspective.visible.is_empty &&
       perspective.overlays.is_empty
 
 
     /* commands */
 
     object Commands
     {
       def apply(commands: Linear_Set[Command]): Commands = new Commands(commands)
       val empty: Commands = apply(Linear_Set.empty)
 
       def starts(commands: Iterator[Command], offset: Text.Offset = 0)
         : Iterator[(Command, Text.Offset)] =
       {
         var i = offset
         for (command <- commands) yield {
           val start = i
           i += command.length
           (command, start)
         }
       }
 
       def starts_pos(commands: Iterator[Command], pos: Token.Pos = Token.Pos.start)
         : Iterator[(Command, Token.Pos)] =
       {
         var p = pos
         for (command <- commands) yield {
           val start = p
           p = (p /: command.span.content)(_.advance(_))
           (command, start)
         }
       }
 
       private val block_size = 256
     }
 
     final class Commands private(val commands: Linear_Set[Command])
     {
       lazy val load_commands: List[Command] =
         commands.iterator.filter(cmd => cmd.blobs.nonEmpty).toList
 
       private lazy val full_index: (Array[(Command, Text.Offset)], Text.Range) =
       {
         val blocks = new mutable.ListBuffer[(Command, Text.Offset)]
         var next_block = 0
         var last_stop = 0
         for ((command, start) <- Commands.starts(commands.iterator)) {
           last_stop = start + command.length
           while (last_stop + 1 > next_block) {
             blocks += (command -> start)
             next_block += Commands.block_size
           }
         }
         (blocks.toArray, Text.Range(0, last_stop))
       }
 
       private def full_range: Text.Range = full_index._2
 
       def iterator(i: Text.Offset = 0): Iterator[(Command, Text.Offset)] =
       {
         if (commands.nonEmpty && full_range.contains(i)) {
           val (cmd0, start0) = full_index._1(i / Commands.block_size)
           Node.Commands.starts(commands.iterator(cmd0), start0) dropWhile {
             case (cmd, start) => start + cmd.length <= i }
         }
         else Iterator.empty
       }
     }
 
     val empty: Node = new Node()
   }
 
   final class Node private(
     val get_blob: Option[Document.Blob] = None,
     val header: Node.Header = Node.no_header,
     val syntax: Option[Outer_Syntax] = None,
     val text_perspective: Text.Perspective = Text.Perspective.empty,
     val perspective: Node.Perspective_Command = Node.no_perspective_command,
     _commands: Node.Commands = Node.Commands.empty)
   {
     def is_empty: Boolean =
       get_blob.isEmpty &&
       header == Node.no_header &&
       text_perspective.is_empty &&
       Node.is_no_perspective_command(perspective) &&
       commands.isEmpty
 
     def has_header: Boolean = header != Node.no_header
 
     override def toString: String =
       if (is_empty) "empty"
       else if (get_blob.isDefined) "blob"
       else "node"
 
     def commands: Linear_Set[Command] = _commands.commands
     def load_commands: List[Command] = _commands.load_commands
     def load_commands_changed(doc_blobs: Blobs): Boolean =
       load_commands.exists(_.blobs_changed(doc_blobs))
 
     def init_blob(blob: Blob): Node = new Node(get_blob = Some(blob.unchanged))
 
     def update_header(new_header: Node.Header): Node =
       new Node(get_blob, new_header, syntax, text_perspective, perspective, _commands)
 
     def update_syntax(new_syntax: Option[Outer_Syntax]): Node =
       new Node(get_blob, header, new_syntax, text_perspective, perspective, _commands)
 
     def update_perspective(
         new_text_perspective: Text.Perspective,
         new_perspective: Node.Perspective_Command): Node =
       new Node(get_blob, header, syntax, new_text_perspective, new_perspective, _commands)
 
     def edit_perspective: Node.Edit[Text.Edit, Text.Perspective] =
       Node.Perspective(perspective.required, text_perspective, perspective.overlays)
 
     def same_perspective(
         other_text_perspective: Text.Perspective,
         other_perspective: Node.Perspective_Command): Boolean =
       text_perspective == other_text_perspective &&
       perspective.required == other_perspective.required &&
       perspective.visible.same(other_perspective.visible) &&
       perspective.overlays == other_perspective.overlays
 
     def update_commands(new_commands: Linear_Set[Command]): Node =
       if (new_commands eq _commands.commands) this
       else
         new Node(get_blob, header, syntax, text_perspective, perspective,
           Node.Commands(new_commands))
 
     def command_iterator(i: Text.Offset = 0): Iterator[(Command, Text.Offset)] =
       _commands.iterator(i)
 
     def command_iterator(range: Text.Range): Iterator[(Command, Text.Offset)] =
       command_iterator(range.start) takeWhile { case (_, start) => start < range.stop }
 
     def command_start(cmd: Command): Option[Text.Offset] =
       Node.Commands.starts(commands.iterator).find(_._1 == cmd).map(_._2)
 
     def source: String =
       get_blob match {
         case Some(blob) => blob.source
         case None => command_iterator(0).map({ case (cmd, _) => cmd.source }).mkString
       }
   }
 
 
   /* development graph */
 
   object Nodes
   {
     val empty: Nodes = new Nodes(Graph.empty(Node.Name.Ordering))
   }
 
   final class Nodes private(graph: Graph[Node.Name, Node])
   {
     def apply(name: Node.Name): Node =
       graph.default_node(name, Node.empty).get_node(name)
 
     def is_suppressed(name: Node.Name): Boolean =
     {
       val graph1 = graph.default_node(name, Node.empty)
       graph1.is_maximal(name) && graph1.get_node(name).is_empty
     }
 
     def purge_suppressed: Option[Nodes] =
       graph.keys_iterator.filter(is_suppressed(_)).toList match {
         case Nil => None
         case del => Some(new Nodes((graph /: del)(_.del_node(_))))
       }
 
     def + (entry: (Node.Name, Node)): Nodes =
     {
       val (name, node) = entry
       val imports = node.header.imports
       val graph1 =
         (graph.default_node(name, Node.empty) /: imports)((g, p) => g.default_node(p, Node.empty))
       val graph2 = (graph1 /: graph1.imm_preds(name))((g, dep) => g.del_edge(dep, name))
       val graph3 = (graph2 /: imports)((g, dep) => g.add_edge(dep, name))
       new Nodes(graph3.map_node(name, _ => node))
     }
 
     def domain: Set[Node.Name] = graph.domain
 
     def iterator: Iterator[(Node.Name, Node)] =
       graph.iterator.map({ case (name, (node, _)) => (name, node) })
 
     def theory_name(theory: String): Option[Node.Name] =
       graph.keys_iterator.find(name => name.theory == theory)
 
     def commands_loading(file_name: Node.Name): List[Command] =
       (for {
         (_, node) <- iterator
         cmd <- node.load_commands.iterator
         name <- cmd.blobs_names.iterator
         if name == file_name
       } yield cmd).toList
 
     def descendants(names: List[Node.Name]): List[Node.Name] = graph.all_succs(names)
+    def requirements(names: List[Node.Name]): List[Node.Name] = graph.all_preds(names).reverse
     def topological_order: List[Node.Name] = graph.topological_order
 
     override def toString: String = topological_order.mkString("Nodes(", ",", ")")
   }
 
 
 
   /** versioning **/
 
   /* particular document versions */
 
   object Version
   {
     val init: Version = new Version()
     def make(nodes: Nodes): Version = new Version(Document_ID.make(), nodes)
 
     def purge_future(versions: Map[Document_ID.Version, Version], future: Future[Version])
       : Future[Version] =
     {
       if (future.is_finished) {
         val version = future.join
         versions.get(version.id) match {
           case Some(version1) if !(version eq version1) => Future.value(version1)
           case _ => future
         }
       }
       else future
     }
 
     def purge_suppressed(
       versions: Map[Document_ID.Version, Version]): Map[Document_ID.Version, Version] =
     {
       (versions /:
         (for ((id, v) <- versions.iterator; v1 <- v.purge_suppressed) yield (id, v1)))(_ + _)
     }
   }
 
   final class Version private(
     val id: Document_ID.Version = Document_ID.none,
     val nodes: Nodes = Nodes.empty)
   {
     override def toString: String = "Version(" + id + ")"
 
     def purge_suppressed: Option[Version] =
       nodes.purge_suppressed.map(new Version(id, _))
   }
 
 
   /* changes of plain text, eventually resulting in document edits */
 
   object Change
   {
     val init: Change = new Change()
 
     def make(previous: Future[Version], edits: List[Edit_Text], version: Future[Version]): Change =
       new Change(Some(previous), edits.reverse, version)
   }
 
   final class Change private(
     val previous: Option[Future[Version]] = Some(Future.value(Version.init)),
     val rev_edits: List[Edit_Text] = Nil,
     val version: Future[Version] = Future.value(Version.init))
   {
     def is_finished: Boolean =
       (previous match { case None => true case Some(future) => future.is_finished }) &&
       version.is_finished
 
     def truncate: Change = new Change(None, Nil, version)
 
     def purge(versions: Map[Document_ID.Version, Version]): Option[Change] =
     {
       val previous1 = previous.map(Version.purge_future(versions, _))
       val version1 = Version.purge_future(versions, version)
       if ((previous eq previous1) && (version eq version1)) None
       else Some(new Change(previous1, rev_edits, version1))
     }
   }
 
 
   /* history navigation */
 
   object History
   {
     val init: History = new History()
   }
 
   final class History private(
     val undo_list: List[Change] = List(Change.init))  // non-empty list
   {
     override def toString: String = "History(" + undo_list.length + ")"
 
     def tip: Change = undo_list.head
     def + (change: Change): History = new History(change :: undo_list)
 
     def prune(check: Change => Boolean, retain: Int): Option[(List[Change], History)] =
     {
       val n = undo_list.iterator.zipWithIndex.find(p => check(p._1)).get._2 + 1
       val (retained, dropped) = undo_list.splitAt(n max retain)
 
       retained.splitAt(retained.length - 1) match {
         case (prefix, List(last)) => Some(dropped, new History(prefix ::: List(last.truncate)))
         case _ => None
       }
     }
 
     def purge(versions: Map[Document_ID.Version, Version]): History =
     {
       val undo_list1 = undo_list.map(_.purge(versions))
       if (undo_list1.forall(_.isEmpty)) this
       else new History(for ((a, b) <- undo_list1 zip undo_list) yield a.getOrElse(b))
     }
   }
 
 
   /* snapshot */
 
   object Snapshot
   {
     val init = State.init.snapshot()
   }
 
   abstract class Snapshot
   {
     def state: State
     def version: Version
     def is_outdated: Boolean
 
     def convert(i: Text.Offset): Text.Offset
     def revert(i: Text.Offset): Text.Offset
     def convert(range: Text.Range): Text.Range
     def revert(range: Text.Range): Text.Range
 
     def node_name: Node.Name
     def node: Node
     def nodes: List[(Node.Name, Node)]
 
     def commands_loading: List[Command]
     def commands_loading_ranges(pred: Node.Name => Boolean): List[Text.Range]
     def current_command(other_node_name: Node.Name, offset: Text.Offset): Option[Command]
 
     def markup_to_XML(range: Text.Range, elements: Markup.Elements): XML.Body
     def messages: List[(XML.Tree, Position.T)]
     def exports: List[Export.Entry]
     def exports_map: Map[String, Export.Entry]
 
     def find_command(id: Document_ID.Generic): Option[(Node, Command)]
     def find_command_position(id: Document_ID.Generic, offset: Symbol.Offset)
       : Option[Line.Node_Position]
 
     def cumulate[A](
       range: Text.Range,
       info: A,
       elements: Markup.Elements,
       result: List[Command.State] => (A, Text.Markup) => Option[A],
       status: Boolean = false): List[Text.Info[A]]
 
     def select[A](
       range: Text.Range,
       elements: Markup.Elements,
       result: List[Command.State] => Text.Markup => Option[A],
       status: Boolean = false): List[Text.Info[A]]
 
     def command_results(range: Text.Range): Command.Results
     def command_results(command: Command): Command.Results
 
     def command_id_map: Map[Document_ID.Generic, Command]
   }
 
 
   /* model */
 
   trait Session
   {
     def resources: Resources
   }
 
   trait Model
   {
     def session: Session
     def is_stable: Boolean
     def snapshot(): Snapshot
 
     def node_name: Node.Name
     def is_theory: Boolean = node_name.is_theory
     override def toString: String = node_name.toString
 
     def get_text(range: Text.Range): Option[String]
 
     def node_required: Boolean
     def get_blob: Option[Blob]
     def bibtex_entries: List[Text.Info[String]]
 
     def node_edits(
       node_header: Node.Header,
       text_edits: List[Text.Edit],
       perspective: Node.Perspective_Text): List[Edit_Text] =
     {
       val edits: List[Node.Edit[Text.Edit, Text.Perspective]] =
         get_blob match {
           case None =>
             List(
               Node.Deps(
                 if (session.resources.session_base.loaded_theory(node_name)) {
                   node_header.append_errors(
                     List("Cannot update finished theory " + quote(node_name.theory)))
                 }
                 else node_header),
               Node.Edits(text_edits), perspective)
           case Some(blob) => List(Node.Blob(blob), Node.Edits(text_edits))
         }
       edits.flatMap(edit => if (edit.is_void) None else Some(node_name -> edit))
     }
   }
 
 
   /** global state -- document structure, execution process, editing history **/
 
   type Assign_Update =
     List[(Document_ID.Command, List[Document_ID.Exec])]  // update of exec state assignment
 
   object State
   {
     class Fail(state: State) extends Exception
 
     object Assignment
     {
       val init: Assignment = new Assignment()
     }
 
     final class Assignment private(
       val command_execs: Map[Document_ID.Command, List[Document_ID.Exec]] = Map.empty,
       val is_finished: Boolean = false)
     {
       override def toString: String = "Assignment(" + command_execs.size + "," + is_finished + ")"
 
       def check_finished: Assignment = { require(is_finished); this }
       def unfinished: Assignment = new Assignment(command_execs, false)
 
       def assign(update: Assign_Update): Assignment =
       {
         require(!is_finished)
         val command_execs1 =
           (command_execs /: update) {
             case (res, (command_id, exec_ids)) =>
               if (exec_ids.isEmpty) res - command_id
               else res + (command_id -> exec_ids)
           }
         new Assignment(command_execs1, true)
       }
     }
 
     val init: State =
       State().define_version(Version.init, Assignment.init).assign(Version.init.id, Nil, Nil)._2
   }
 
   final case class State private(
     /*reachable versions*/
     versions: Map[Document_ID.Version, Version] = Map.empty,
     /*inlined auxiliary files*/
     blobs: Set[SHA1.Digest] = Set.empty,
     /*static markup from define_command*/
     commands: Map[Document_ID.Command, Command.State] = Map.empty,
     /*dynamic markup from execution*/
     execs: Map[Document_ID.Exec, Command.State] = Map.empty,
     /*command-exec assignment for each version*/
     assignments: Map[Document_ID.Version, State.Assignment] = Map.empty,
     /*commands with markup produced by other commands (imm_succs)*/
     commands_redirection: Graph[Document_ID.Command, Unit] = Graph.long,
     /*explicit (linear) history*/
     history: History = History.init,
     /*intermediate state between remove_versions/removed_versions*/
     removing_versions: Boolean = false)
   {
     override def toString: String =
       "State(versions = " + versions.size +
       ", blobs = " + blobs.size +
       ", commands = " + commands.size +
       ", execs = " + execs.size +
       ", assignments = " + assignments.size +
       ", commands_redirection = " + commands_redirection.size +
       ", history = " + history.undo_list.size +
       ", removing_versions = " + removing_versions + ")"
 
     private def fail[A]: A = throw new State.Fail(this)
 
     def define_version(version: Version, assignment: State.Assignment): State =
     {
       val id = version.id
       copy(versions = versions + (id -> version),
         assignments = assignments + (id -> assignment.unfinished))
     }
 
     def define_blob(digest: SHA1.Digest): State = copy(blobs = blobs + digest)
     def defined_blob(digest: SHA1.Digest): Boolean = blobs.contains(digest)
 
     def define_command(command: Command): State =
     {
       val id = command.id
       copy(commands = commands + (id -> command.init_state))
     }
 
     def defined_command(id: Document_ID.Command): Boolean = commands.isDefinedAt(id)
 
     def the_version(id: Document_ID.Version): Version = versions.getOrElse(id, fail)
     def the_static_state(id: Document_ID.Command): Command.State = commands.getOrElse(id, fail)
     def the_dynamic_state(id: Document_ID.Exec): Command.State = execs.getOrElse(id, fail)
     def the_assignment(version: Version): State.Assignment = assignments.getOrElse(version.id, fail)
 
     def lookup_id(id: Document_ID.Generic): Option[Command.State] =
       commands.get(id) orElse execs.get(id)
 
     private def self_id(st: Command.State)(id: Document_ID.Generic): Boolean =
       id == st.command.id ||
       (execs.get(id) match { case Some(st1) => st1.command.id == st.command.id case None => false })
 
     private def other_id(id: Document_ID.Generic)
       : Option[(Symbol.Text_Chunk.Id, Symbol.Text_Chunk)] =
       lookup_id(id).map(st => ((Symbol.Text_Chunk.Id(st.command.id), st.command.chunk)))
 
     private def redirection(st: Command.State): Graph[Document_ID.Command, Unit] =
       (commands_redirection /: st.markups.redirection_iterator)({ case (graph, id) =>
         graph.default_node(id, ()).default_node(st.command.id, ()).add_edge(id, st.command.id) })
 
     def accumulate(id: Document_ID.Generic, message: XML.Elem, xml_cache: XML.Cache)
       : (Command.State, State) =
     {
       execs.get(id) match {
         case Some(st) =>
           val new_st = st.accumulate(self_id(st), other_id _, message, xml_cache)
           val execs1 = execs + (id -> new_st)
           (new_st, copy(execs = execs1, commands_redirection = redirection(new_st)))
         case None =>
           commands.get(id) match {
             case Some(st) =>
               val new_st = st.accumulate(self_id(st), other_id _, message, xml_cache)
               val commands1 = commands + (id -> new_st)
               (new_st, copy(commands = commands1, commands_redirection = redirection(new_st)))
             case None => fail
           }
       }
     }
 
     def add_export(id: Document_ID.Generic, entry: Command.Exports.Entry): (Command.State, State) =
     {
       execs.get(id) match {
         case Some(st) =>
           st.add_export(entry) match {
             case Some(new_st) => (new_st, copy(execs = execs + (id -> new_st)))
             case None => fail
           }
         case None =>
           commands.get(id) match {
             case Some(st) =>
               st.add_export(entry) match {
                 case Some(new_st) => (new_st, copy(commands = commands + (id -> new_st)))
                 case None => fail
               }
             case None => fail
           }
       }
     }
 
     def node_exports(version: Version, node_name: Node.Name): Command.Exports =
       Command.Exports.merge(
         for {
           command <- version.nodes(node_name).commands.iterator
           st <- command_states(version, command).iterator
         } yield st.exports)
 
     def assign(id: Document_ID.Version, edited: List[String], update: Assign_Update)
       : ((List[Node.Name], List[Command]), State) =
     {
       val version = the_version(id)
 
       val edited_set = edited.toSet
       val edited_nodes =
         (for { (name, _) <- version.nodes.iterator if edited_set(name.node) } yield name).toList
 
       def upd(exec_id: Document_ID.Exec, st: Command.State)
           : Option[(Document_ID.Exec, Command.State)] =
         if (execs.isDefinedAt(exec_id)) None else Some(exec_id -> st)
 
       val (changed_commands, new_execs) =
         ((Nil: List[Command], execs) /: update) {
           case ((commands1, execs1), (command_id, exec)) =>
             val st = the_static_state(command_id)
             val command = st.command
             val commands2 = command :: commands1
             val execs2 =
               exec match {
                 case Nil => execs1
                 case eval_id :: print_ids =>
                   execs1 ++ upd(eval_id, st) ++
                     (for (id <- print_ids; up <- upd(id, command.empty_state)) yield up)
               }
             (commands2, execs2)
         }
       val new_assignment = the_assignment(version).assign(update)
       val new_state = copy(assignments = assignments + (id -> new_assignment), execs = new_execs)
 
       ((edited_nodes, changed_commands), new_state)
     }
 
     def is_assigned(version: Version): Boolean =
       assignments.get(version.id) match {
         case Some(assgn) => assgn.is_finished
         case None => false
       }
 
     def is_stable(change: Change): Boolean =
       change.is_finished && is_assigned(change.version.get_finished)
 
     def recent_finished: Change = history.undo_list.find(_.is_finished) getOrElse fail
     def recent_stable: Change = history.undo_list.find(is_stable) getOrElse fail
     def stable_tip_version: Option[Version] =
       if (is_stable(history.tip)) Some(history.tip.version.get_finished) else None
 
     def continue_history(
       previous: Future[Version],
       edits: List[Edit_Text],
       version: Future[Version]): State =
     {
       val change = Change.make(previous, edits, version)
       copy(history = history + change)
     }
 
     def remove_versions(retain: Int = 0): (List[Version], State) =
     {
       history.prune(is_stable, retain) match {
         case Some((dropped, history1)) =>
           val old_versions = dropped.map(change => change.version.get_finished)
           val removing = old_versions.nonEmpty
           val state1 = copy(history = history1, removing_versions = removing)
           (old_versions, state1)
         case None => fail
       }
     }
 
     def removed_versions(removed: List[Document_ID.Version]): State =
     {
       val versions1 = Version.purge_suppressed(versions -- removed)
 
       val assignments1 = assignments -- removed
       var blobs1_names = Set.empty[Node.Name]
       var blobs1 = Set.empty[SHA1.Digest]
       var commands1 = Map.empty[Document_ID.Command, Command.State]
       var execs1 = Map.empty[Document_ID.Exec, Command.State]
       for {
         (version_id, version) <- versions1.iterator
         command_execs = assignments1(version_id).command_execs
         (_, node) <- version.nodes.iterator
         command <- node.commands.iterator
       } {
         for ((name, digest) <- command.blobs_defined) {
           blobs1_names += name
           blobs1 += digest
         }
 
         if (!commands1.isDefinedAt(command.id))
           commands.get(command.id).foreach(st => commands1 += (command.id -> st))
 
         for (exec_id <- command_execs.getOrElse(command.id, Nil)) {
           if (!execs1.isDefinedAt(exec_id))
             execs.get(exec_id).foreach(st => execs1 += (exec_id -> st))
         }
       }
 
       copy(
         versions = versions1,
         blobs = blobs1,
         commands = commands1,
         execs = execs1,
         commands_redirection = commands_redirection.restrict(commands1.keySet),
         assignments = assignments1,
         history = history.purge(versions1),
         removing_versions = false)
     }
 
     def command_id_map(version: Version, commands: Iterable[Command])
       : Map[Document_ID.Generic, Command] =
     {
       require(is_assigned(version))
       val assignment = the_assignment(version).check_finished
       (for {
         command <- commands.iterator
         id <- (command.id :: assignment.command_execs.getOrElse(command.id, Nil)).iterator
       } yield (id -> command)).toMap
     }
 
     def command_maybe_consolidated(version: Version, command: Command): Boolean =
     {
       require(is_assigned(version))
       try {
         the_assignment(version).check_finished.command_execs.getOrElse(command.id, Nil) match {
           case eval_id :: print_ids =>
             the_dynamic_state(eval_id).maybe_consolidated &&
             !print_ids.exists(print_id => the_dynamic_state(print_id).consolidating)
           case Nil => false
         }
       }
       catch { case _: State.Fail => false }
     }
 
     private def command_states_self(version: Version, command: Command)
       : List[(Document_ID.Generic, Command.State)] =
     {
       require(is_assigned(version))
       try {
         the_assignment(version).check_finished.command_execs.getOrElse(command.id, Nil)
           .map(id => id -> the_dynamic_state(id)) match {
             case Nil => fail
             case res => res
           }
       }
       catch {
         case _: State.Fail =>
           try { List(command.id -> the_static_state(command.id)) }
           catch { case _: State.Fail => List(command.id -> command.init_state) }
       }
     }
 
     def command_states(version: Version, command: Command): List[Command.State] =
     {
       val self = command_states_self(version, command)
       val others =
         if (commands_redirection.defined(command.id)) {
           (for {
             command_id <- commands_redirection.imm_succs(command.id).iterator
             (id, st) <- command_states_self(version, the_static_state(command_id).command)
             if !self.exists(_._1 == id)
           } yield (id, st)).toMap.valuesIterator.toList
         }
         else Nil
       self.map(_._2) ::: others.flatMap(_.redirect(command))
     }
 
     def command_results(version: Version, command: Command): Command.Results =
       Command.State.merge_results(command_states(version, command))
 
     def command_markup(version: Version, command: Command, index: Command.Markup_Index,
         range: Text.Range, elements: Markup.Elements): Markup_Tree =
       Command.State.merge_markup(command_states(version, command), index, range, elements)
 
     def markup_to_XML(
       version: Version,
       node_name: Node.Name,
       range: Text.Range,
       elements: Markup.Elements): XML.Body =
     {
       val node = version.nodes(node_name)
       if (node_name.is_theory) {
         val markup_index = Command.Markup_Index.markup
         (for {
           command <- node.commands.iterator
           command_range <- command.range.try_restrict(range).iterator
           markup = command_markup(version, command, markup_index, command_range, elements)
           tree <- markup.to_XML(command_range, command.source, elements).iterator
         } yield tree).toList
       }
       else {
         val node_source = node.source
         Text.Range(0, node_source.length).try_restrict(range) match {
           case None => Nil
           case Some(node_range) =>
             val markup =
               version.nodes.commands_loading(node_name).headOption match {
                 case None => Markup_Tree.empty
                 case Some(command) =>
                   val chunk_name = Symbol.Text_Chunk.File(node_name.node)
                   val markup_index = Command.Markup_Index(false, chunk_name)
                   command_markup(version, command, markup_index, node_range, elements)
               }
             markup.to_XML(node_range, node_source, elements)
         }
       }
     }
 
     def node_initialized(version: Version, name: Node.Name): Boolean =
       name.is_theory &&
       (version.nodes(name).commands.iterator.find(_.potentially_initialized) match {
         case None => false
         case Some(command) => command_states(version, command).headOption.exists(_.initialized)
       })
 
     def node_maybe_consolidated(version: Version, name: Node.Name): Boolean =
       name.is_theory &&
       version.nodes(name).commands.reverse.iterator.forall(command_maybe_consolidated(version, _))
 
     def node_consolidated(version: Version, name: Node.Name): Boolean =
       !name.is_theory ||
       {
         val it = version.nodes(name).commands.reverse.iterator
         it.hasNext && command_states(version, it.next).exists(_.consolidated)
       }
 
     // persistent user-view
     def snapshot(name: Node.Name = Node.Name.empty, pending_edits: List[Text.Edit] = Nil)
       : Snapshot =
     {
       val stable = recent_stable
       val latest = history.tip
 
 
       /* pending edits and unstable changes */
 
       val rev_pending_changes =
         for {
           change <- history.undo_list.takeWhile(_ != stable)
           (a, edits) <- change.rev_edits
           if a == name
         } yield edits
 
       val edits =
         (pending_edits /: rev_pending_changes)({
           case (edits, Node.Edits(es)) => es ::: edits
           case (edits, _) => edits
         })
       lazy val reverse_edits = edits.reverse
 
       new Snapshot
       {
         /* global information */
 
         val state: State = State.this
         val version: Version = stable.version.get_finished
         val is_outdated: Boolean = pending_edits.nonEmpty || latest != stable
 
 
         /* local node content */
 
         def convert(offset: Text.Offset) = (offset /: edits)((i, edit) => edit.convert(i))
         def revert(offset: Text.Offset) = (offset /: reverse_edits)((i, edit) => edit.revert(i))
 
         def convert(range: Text.Range) = range.map(convert(_))
         def revert(range: Text.Range) = range.map(revert(_))
 
         val node_name: Node.Name = name
         val node: Node = version.nodes(name)
 
         def nodes: List[(Node.Name, Node)] =
           (node_name :: node.load_commands.flatMap(_.blobs_names)).
             map(name => (name, version.nodes(name)))
 
         val commands_loading: List[Command] =
           if (node_name.is_theory) Nil
           else version.nodes.commands_loading(node_name)
 
         def commands_loading_ranges(pred: Node.Name => Boolean): List[Text.Range] =
           (for {
             cmd <- node.load_commands.iterator
             blob_name <- cmd.blobs_names.iterator
             if pred(blob_name)
             start <- node.command_start(cmd)
           } yield convert(cmd.core_range + start)).toList
 
         def current_command(other_node_name: Node.Name, offset: Text.Offset): Option[Command] =
           if (other_node_name.is_theory) {
             val other_node = version.nodes(other_node_name)
             val iterator = other_node.command_iterator(revert(offset) max 0)
             if (iterator.hasNext) {
               val (command0, _) = iterator.next
               other_node.commands.reverse.iterator(command0).find(command => !command.is_ignored)
             }
             else other_node.commands.reverse.iterator.find(command => !command.is_ignored)
           }
           else version.nodes.commands_loading(other_node_name).headOption
 
         def markup_to_XML(range: Text.Range, elements: Markup.Elements): XML.Body =
           state.markup_to_XML(version, node_name, range, elements)
 
         lazy val messages: List[(XML.Tree, Position.T)] =
           (for {
             (command, start) <-
               Document.Node.Commands.starts_pos(
                 node.commands.iterator, Token.Pos.file(node_name.node))
             pos = command.span.keyword_pos(start).position(command.span.name)
             (_, tree) <- state.command_results(version, command).iterator
            } yield (tree, pos)).toList
 
         lazy val exports: List[Export.Entry] =
           state.node_exports(version, node_name).iterator.map(_._2).toList
 
         lazy val exports_map: Map[String, Export.Entry] =
           (for (entry <- exports.iterator) yield (entry.name, entry)).toMap
 
 
         /* find command */
 
         def find_command(id: Document_ID.Generic): Option[(Node, Command)] =
           state.lookup_id(id) match {
             case None => None
             case Some(st) =>
               val command = st.command
               val node = version.nodes(command.node_name)
               if (node.commands.contains(command)) Some((node, command)) else None
           }
 
         def find_command_position(id: Document_ID.Generic, offset: Symbol.Offset)
             : Option[Line.Node_Position] =
           for ((node, command) <- find_command(id))
           yield {
             val name = command.node_name.node
             val sources_iterator =
               node.commands.iterator.takeWhile(_ != command).map(_.source) ++
                 (if (offset == 0) Iterator.empty
                  else Iterator.single(command.source(Text.Range(0, command.chunk.decode(offset)))))
             val pos = (Line.Position.zero /: sources_iterator)(_.advance(_))
             Line.Node_Position(name, pos)
           }
 
 
         /* cumulate markup */
 
         def cumulate[A](
           range: Text.Range,
           info: A,
           elements: Markup.Elements,
           result: List[Command.State] => (A, Text.Markup) => Option[A],
           status: Boolean = false): List[Text.Info[A]] =
         {
           val former_range = revert(range).inflate_singularity
           val (chunk_name, command_iterator) =
             commands_loading.headOption match {
               case None => (Symbol.Text_Chunk.Default, node.command_iterator(former_range))
               case Some(command) => (Symbol.Text_Chunk.File(node_name.node), Iterator((command, 0)))
             }
           val markup_index = Command.Markup_Index(status, chunk_name)
           (for {
             (command, command_start) <- command_iterator
             chunk <- command.chunks.get(chunk_name).iterator
             states = state.command_states(version, command)
             res = result(states)
             markup_range <- (former_range - command_start).try_restrict(chunk.range).iterator
             markup = Command.State.merge_markup(states, markup_index, markup_range, elements)
             Text.Info(r0, a) <- markup.cumulate[A](markup_range, info, elements,
               {
                 case (a, Text.Info(r0, b)) => res(a, Text.Info(convert(r0 + command_start), b))
               }).iterator
             r1 <- convert(r0 + command_start).try_restrict(range).iterator
           } yield Text.Info(r1, a)).toList
         }
 
         def select[A](
           range: Text.Range,
           elements: Markup.Elements,
           result: List[Command.State] => Text.Markup => Option[A],
           status: Boolean = false): List[Text.Info[A]] =
         {
           def result1(states: List[Command.State]): (Option[A], Text.Markup) => Option[Option[A]] =
           {
             val res = result(states)
             (_: Option[A], x: Text.Markup) =>
               res(x) match {
                 case None => None
                 case some => Some(some)
               }
           }
           for (Text.Info(r, Some(x)) <- cumulate(range, None, elements, result1 _, status))
             yield Text.Info(r, x)
         }
 
 
         /* command results */
 
         def command_results(range: Text.Range): Command.Results =
           Command.State.merge_results(
             select[List[Command.State]](range, Markup.Elements.full, command_states =>
               { case _ => Some(command_states) }).flatMap(_.info))
 
         def command_results(command: Command): Command.Results =
           state.command_results(version, command)
 
 
         /* command ids: static and dynamic */
 
         def command_id_map: Map[Document_ID.Generic, Command] =
           state.command_id_map(version, version.nodes(node_name).commands)
 
 
         /* output */
 
         override def toString: String =
           "Snapshot(node = " + node_name.node + ", version = " + version.id +
             (if (is_outdated) ", outdated" else "") + ")"
       }
     }
   }
 }