diff --git a/src/Pure/Thy/thy_header.scala b/src/Pure/Thy/thy_header.scala
--- a/src/Pure/Thy/thy_header.scala
+++ b/src/Pure/Thy/thy_header.scala
@@ -1,275 +1,275 @@
 /*  Title:      Pure/Thy/thy_header.scala
     Author:     Makarius
 
 Static theory header information.
 */
 
 package isabelle
 
 
 import scala.util.parsing.input.Reader
 import scala.util.matching.Regex
 
 
 object Thy_Header
 {
   /* bootstrap keywords */
 
   type Keywords = List[(String, Keyword.Spec)]
   type Abbrevs = List[(String, String)]
 
   val CHAPTER = "chapter"
   val SECTION = "section"
   val SUBSECTION = "subsection"
   val SUBSUBSECTION = "subsubsection"
   val PARAGRAPH = "paragraph"
   val SUBPARAGRAPH = "subparagraph"
   val TEXT = "text"
   val TXT = "txt"
   val TEXT_RAW = "text_raw"
 
   val THEORY = "theory"
   val IMPORTS = "imports"
   val KEYWORDS = "keywords"
   val ABBREVS = "abbrevs"
   val AND = "and"
   val BEGIN = "begin"
 
   val bootstrap_header: Keywords =
     List(
       ("%", Keyword.Spec()),
       ("(", Keyword.Spec()),
       (")", Keyword.Spec()),
       (",", Keyword.Spec()),
       ("::", Keyword.Spec()),
       ("=", Keyword.Spec()),
       (AND, Keyword.Spec()),
       (BEGIN, Keyword.Spec(kind = Keyword.QUASI_COMMAND)),
       (IMPORTS, Keyword.Spec(kind = Keyword.QUASI_COMMAND)),
       (KEYWORDS, Keyword.Spec(kind = Keyword.QUASI_COMMAND)),
       (ABBREVS, Keyword.Spec(kind = Keyword.QUASI_COMMAND)),
       (CHAPTER, Keyword.Spec(kind = Keyword.DOCUMENT_HEADING)),
       (SECTION, Keyword.Spec(kind = Keyword.DOCUMENT_HEADING)),
       (SUBSECTION, Keyword.Spec(kind = Keyword.DOCUMENT_HEADING)),
       (SUBSUBSECTION, Keyword.Spec(kind = Keyword.DOCUMENT_HEADING)),
       (PARAGRAPH, Keyword.Spec(kind = Keyword.DOCUMENT_HEADING)),
       (SUBPARAGRAPH, Keyword.Spec(kind = Keyword.DOCUMENT_HEADING)),
       (TEXT, Keyword.Spec(kind = Keyword.DOCUMENT_BODY)),
       (TXT, Keyword.Spec(kind = Keyword.DOCUMENT_BODY)),
       (TEXT_RAW, Keyword.Spec(kind = Keyword.DOCUMENT_RAW)),
       (THEORY, Keyword.Spec(kind = Keyword.THY_BEGIN, tags = List("theory"))),
       ("ML", Keyword.Spec(kind = Keyword.THY_DECL, tags = List("ML"))))
 
   private val bootstrap_keywords =
     Keyword.Keywords.empty.add_keywords(bootstrap_header)
 
   val bootstrap_syntax: Outer_Syntax =
     Outer_Syntax.empty.add_keywords(bootstrap_header)
 
 
   /* file name vs. theory name */
 
   val PURE = "Pure"
   val ML_BOOTSTRAP = "ML_Bootstrap"
   val ml_roots = List("ROOT0.ML" -> "ML_Root0", "ROOT.ML" -> "ML_Root")
   val bootstrap_thys = List(PURE, ML_BOOTSTRAP).map(a => a -> ("Bootstrap_" + a))
 
   val bootstrap_global_theories =
     (Sessions.root_name :: (ml_roots ::: bootstrap_thys).map(_._2)).map(_ -> PURE)
 
   private val Thy_File_Name = new Regex(""".*?([^/\\:]+)\.thy""")
   private val Split_File_Name = new Regex("""(.*?)[/\\]*([^/\\:]+)""")
   private val File_Name = new Regex(""".*?([^/\\:]+)""")
 
   def is_base_name(s: String): Boolean =
     s != "" && !s.exists("/\\:".contains(_))
 
   def split_file_name(s: String): Option[(String, String)] =
     s match {
       case Split_File_Name(s1, s2) => Some((s1, s2))
       case _ => None
     }
 
   def import_name(s: String): String =
     s match {
       case File_Name(name) if !name.endsWith(".thy") => name
       case _ => error("Malformed theory import: " + quote(s))
     }
 
   def theory_name(s: String): String =
     s match {
       case Thy_File_Name(name) => bootstrap_name(name)
       case File_Name(name) =>
         if (name == Sessions.root_name) name
         else ml_roots.collectFirst({ case (a, b) if a == name => b }).getOrElse("")
       case _ => ""
     }
 
   def is_ml_root(theory: String): Boolean =
     ml_roots.exists({ case (_, b) => b == theory })
 
   def is_bootstrap(theory: String): Boolean =
     bootstrap_thys.exists({ case (_, b) => b == theory })
 
   def bootstrap_name(theory: String): String =
     bootstrap_thys.collectFirst({ case (a, b) if a == theory => b }).getOrElse(theory)
 
   def try_read_dir(dir: Path): List[String] =
   {
     val files =
       try { File.read_dir(dir) }
       catch { case ERROR(_) => Nil }
     for { Thy_File_Name(name) <- files } yield name
   }
 
 
   /* parser */
 
   trait Parser extends Parse.Parser
   {
     val header: Parser[Thy_Header] =
     {
       val load_command =
         ($$$("(") ~! (position(name) <~ $$$(")")) ^^ { case _ ~ x => x }) |
           success(("", Position.none))
 
       val keyword_kind = atom("outer syntax keyword specification", _.is_name)
       val keyword_spec =
         position(keyword_kind) ~ load_command ~ tags ^^
           { case (a, b) ~ c ~ d =>
               Keyword.Spec(kind = a, kind_pos = b,
                 load_command = c._1, load_command_pos = c._2, tags = d)
           }
 
       val keyword_decl =
         rep1(string) ~
         opt($$$("::") ~! keyword_spec ^^ { case _ ~ x => x }) ^^
         { case xs ~ y => xs.map((_, y.getOrElse(Keyword.Spec()))) }
 
       val keyword_decls =
         keyword_decl ~ rep($$$(AND) ~! keyword_decl ^^ { case _ ~ x => x }) ^^
         { case xs ~ yss => (xs :: yss).flatten }
 
       val abbrevs =
         rep1sep(rep1(text) ~ ($$$("=") ~! rep1(text)), $$$("and")) ^^
           { case res => for ((as ~ (_ ~ bs)) <- res; a <- as; b <- bs) yield (a, b) }
 
       val args =
         position(theory_name) ~
         (opt($$$(IMPORTS) ~! rep1(position(theory_name))) ^^
           { case None => Nil case Some(_ ~ xs) => xs }) ~
         (opt($$$(KEYWORDS) ~! keyword_decls) ^^
           { case None => Nil case Some(_ ~ xs) => xs }) ~
         (opt($$$(ABBREVS) ~! abbrevs) ^^
           { case None => Nil case Some(_ ~ xs) => xs }) ~
         $$$(BEGIN) ^^ { case a ~ b ~ c ~ d ~ _ => Thy_Header(a._1, a._2, b, c, d) }
 
       val heading =
         (command(CHAPTER) |
           command(SECTION) |
           command(SUBSECTION) |
           command(SUBSUBSECTION) |
           command(PARAGRAPH) |
           command(SUBPARAGRAPH) |
           command(TEXT) |
           command(TXT) |
           command(TEXT_RAW)) ~
         annotation ~! document_source
 
       (rep(heading) ~ command(THEORY) ~ annotation) ~! args ^^ { case _ ~ x => x }
     }
   }
 
 
   /* read -- lazy scanning */
 
   private def read_tokens(reader: Reader[Char], strict: Boolean): (List[Token], List[Token]) =
   {
     val token = Token.Parsers.token(bootstrap_keywords)
-    def make_tokens(in: Reader[Char]): Stream[Token] =
+    def make_tokens(in: Reader[Char]): LazyList[Token] =
       token(in) match {
         case Token.Parsers.Success(tok, rest) => tok #:: make_tokens(rest)
-        case _ => Stream.empty
+        case _ => LazyList.empty
       }
 
     val all_tokens = make_tokens(reader)
     val drop_tokens =
       if (strict) Nil
       else all_tokens.takeWhile(tok => !tok.is_command(Thy_Header.THEORY)).toList
 
     val tokens = all_tokens.drop(drop_tokens.length)
     val tokens1 = tokens.takeWhile(tok => !tok.is_begin).toList
     val tokens2 = tokens.dropWhile(tok => !tok.is_begin).headOption.toList
 
     (drop_tokens, tokens1 ::: tokens2)
   }
 
   private object Parser extends Parser
   {
     def parse_header(tokens: List[Token], pos: Token.Pos): Thy_Header =
       parse(commit(header), Token.reader(tokens, pos)) match {
         case Success(result, _) => result
         case bad => error(bad.toString)
       }
   }
 
   def read(node_name: Document.Node.Name, reader: Reader[Char],
     command: Boolean = true,
     strict: Boolean = true): Thy_Header =
   {
     val (_, tokens0) = read_tokens(reader, true)
     val text = Scan.reader_decode_utf8(reader, Token.implode(tokens0))
 
     val (skip_tokens, tokens) = read_tokens(Scan.char_reader(text), strict)
     val pos =
       if (command) Token.Pos.command
       else skip_tokens.foldLeft(Token.Pos.file(node_name.node))(_ advance _)
 
     Parser.parse_header(tokens, pos).map(Symbol.decode).check(node_name)
   }
 }
 
 sealed case class Thy_Header(
   name: String,
   pos: Position.T,
   imports: List[(String, Position.T)],
   keywords: Thy_Header.Keywords,
   abbrevs: Thy_Header.Abbrevs)
 {
   def map(f: String => String): Thy_Header =
     Thy_Header(f(name), pos,
       imports.map({ case (a, b) => (f(a), b) }),
       keywords.map({ case (a, spec) => (f(a), spec.map(f)) }),
       abbrevs.map({ case (a, b) => (f(a), f(b)) }))
 
   def check(node_name: Document.Node.Name): Thy_Header =
   {
     val base_name = node_name.theory_base_name
     if (Long_Name.is_qualified(name)) {
       error("Bad theory name " + quote(name) + " with qualification" + Position.here(pos))
     }
     if (base_name != name) {
       error("Bad theory name " + quote(name) + " for file " + Path.basic(base_name).thy +
         Position.here(pos) + Completion.report_theories(pos, List(base_name)))
     }
 
     for ((_, spec) <- keywords) {
       if (spec.kind != Keyword.THY_LOAD && spec.load_command.nonEmpty) {
         error("Illegal load command specification for kind: " + quote(spec.kind) +
           Position.here(spec.kind_pos))
       }
       if (!Command_Span.load_commands.exists(_.name == spec.load_command)) {
         val completion =
           for {
             load_command <- Command_Span.load_commands
             name = load_command.name
             if name.startsWith(spec.load_command)
           } yield (name, (Markup.LOAD_COMMAND, name))
         error("Unknown load command specification: " + quote(spec.load_command) +
           Position.here(spec.load_command_pos) +
           Completion.report_names(spec.load_command_pos, completion))
       }
     }
     this
   }
 }
diff --git a/src/Pure/Thy/thy_syntax.scala b/src/Pure/Thy/thy_syntax.scala
--- a/src/Pure/Thy/thy_syntax.scala
+++ b/src/Pure/Thy/thy_syntax.scala
@@ -1,362 +1,362 @@
 /*  Title:      Pure/Thy/thy_syntax.scala
     Author:     Makarius
 
 Superficial theory syntax: tokens and spans.
 */
 
 package isabelle
 
 
 import scala.collection.mutable
 import scala.annotation.tailrec
 
 
 object Thy_Syntax
 {
   /** perspective **/
 
   def command_perspective(
       node: Document.Node,
       perspective: Text.Perspective,
       overlays: Document.Node.Overlays): (Command.Perspective, Command.Perspective) =
   {
     if (perspective.is_empty && overlays.is_empty)
       (Command.Perspective.empty, Command.Perspective.empty)
     else {
       val has_overlay = overlays.commands
       val visible = new mutable.ListBuffer[Command]
       val visible_overlay = new mutable.ListBuffer[Command]
       @tailrec
-      def check_ranges(ranges: List[Text.Range], commands: Stream[(Command, Text.Offset)]): Unit =
+      def check_ranges(ranges: List[Text.Range], commands: LazyList[(Command, Text.Offset)]): Unit =
       {
         (ranges, commands) match {
           case (range :: more_ranges, (command, offset) #:: more_commands) =>
             val command_range = command.range + offset
             range compare command_range match {
               case 0 =>
                 visible += command
                 visible_overlay += command
                 check_ranges(ranges, more_commands)
               case c =>
                 if (has_overlay(command)) visible_overlay += command
 
                 if (c < 0) check_ranges(more_ranges, commands)
                 else check_ranges(ranges, more_commands)
             }
 
           case (Nil, (command, _) #:: more_commands) =>
             if (has_overlay(command)) visible_overlay += command
 
             check_ranges(Nil, more_commands)
 
           case _ =>
         }
       }
 
       val commands =
         (if (overlays.is_empty) node.command_iterator(perspective.range)
-         else node.command_iterator()).toStream
+         else node.command_iterator()).to(LazyList)
       check_ranges(perspective.ranges, commands)
       (Command.Perspective(visible.toList), Command.Perspective(visible_overlay.toList))
     }
   }
 
 
 
   /** header edits: graph structure and outer syntax **/
 
   private def header_edits(
     resources: Resources,
     previous: Document.Version,
     edits: List[Document.Edit_Text]):
     (List[Document.Node.Name], Document.Nodes, List[Document.Edit_Command]) =
   {
     val syntax_changed0 = new mutable.ListBuffer[Document.Node.Name]
     var nodes = previous.nodes
     val doc_edits = new mutable.ListBuffer[Document.Edit_Command]
 
     edits foreach {
       case (name, Document.Node.Deps(header)) =>
         val node = nodes(name)
         val update_header =
           node.header.errors.nonEmpty || header.errors.nonEmpty || node.header != header
         if (update_header) {
           val node1 = node.update_header(header)
           if (node.header.imports != node1.header.imports ||
               node.header.keywords != node1.header.keywords ||
               node.header.abbrevs != node1.header.abbrevs ||
               node.header.errors != node1.header.errors) syntax_changed0 += name
           nodes += (name -> node1)
           doc_edits += (name -> Document.Node.Deps(header))
         }
       case _ =>
     }
 
     val syntax_changed = nodes.descendants(syntax_changed0.toList)
 
     for (name <- syntax_changed) {
       val node = nodes(name)
       val syntax =
         if (node.is_empty) None
         else {
           val header = node.header
           val imports_syntax =
             if (header.imports.nonEmpty) {
               Outer_Syntax.merge(header.imports.map(resources.session_base.node_syntax(nodes, _)))
             }
             else resources.session_base.overall_syntax
           Some(imports_syntax + header)
         }
       nodes += (name -> node.update_syntax(syntax))
     }
 
     (syntax_changed, nodes, doc_edits.toList)
   }
 
 
 
   /** text edits **/
 
   /* edit individual command source */
 
   @tailrec def edit_text(eds: List[Text.Edit], commands: Linear_Set[Command]): Linear_Set[Command] =
   {
     eds match {
       case e :: es =>
         def insert_text(cmd: Option[Command], text: String): Linear_Set[Command] =
           if (text == "") commands else commands.insert_after(cmd, Command.text(text))
 
         Document.Node.Commands.starts(commands.iterator).find {
           case (cmd, cmd_start) =>
             e.can_edit(cmd.source, cmd_start) ||
               e.is_insert && e.start == cmd_start + cmd.length
         } match {
           case Some((cmd, cmd_start)) if e.can_edit(cmd.source, cmd_start) =>
             val (rest, text) = e.edit(cmd.source, cmd_start)
             val new_commands = insert_text(Some(cmd), text) - cmd
             edit_text(rest.toList ::: es, new_commands)
 
           case Some((cmd, _)) =>
             edit_text(es, insert_text(Some(cmd), e.text))
 
           case None =>
             require(e.is_insert && e.start == 0, "bad text edit")
             edit_text(es, insert_text(None, e.text))
         }
       case Nil => commands
     }
   }
 
 
   /* reparse range of command spans */
 
   @tailrec private def chop_common(
       cmds: List[Command],
       blobs_spans: List[(Command.Blobs_Info, Command_Span.Span)])
     : (List[Command], List[(Command.Blobs_Info, Command_Span.Span)]) =
   {
     (cmds, blobs_spans) match {
       case (cmd :: cmds, (blobs_info, span) :: rest)
       if cmd.blobs_info == blobs_info && cmd.span == span => chop_common(cmds, rest)
       case _ => (cmds, blobs_spans)
     }
   }
 
   private def reparse_spans(
     resources: Resources,
     syntax: Outer_Syntax,
     get_blob: Document.Node.Name => Option[Document.Blob],
     can_import: Document.Node.Name => Boolean,
     node_name: Document.Node.Name,
     commands: Linear_Set[Command],
     first: Command, last: Command): Linear_Set[Command] =
   {
     val cmds0 = commands.iterator(first, last).toList
     val blobs_spans0 =
       syntax.parse_spans(cmds0.iterator.map(_.source).mkString).map(span =>
         (Command.blobs_info(resources, syntax, get_blob, can_import, node_name, span), span))
 
     val (cmds1, blobs_spans1) = chop_common(cmds0, blobs_spans0)
 
     val (rev_cmds2, rev_blobs_spans2) = chop_common(cmds1.reverse, blobs_spans1.reverse)
     val cmds2 = rev_cmds2.reverse
     val blobs_spans2 = rev_blobs_spans2.reverse
 
     cmds2 match {
       case Nil =>
         assert(blobs_spans2.isEmpty)
         commands
       case cmd :: _ =>
         val hook = commands.prev(cmd)
         val inserted =
           blobs_spans2.map({ case (blobs, span) =>
             Command(Document_ID.make(), node_name, blobs, span) })
         cmds2.foldLeft(commands)(_ - _).append_after(hook, inserted)
     }
   }
 
 
   /* main */
 
   def diff_commands(old_cmds: Linear_Set[Command], new_cmds: Linear_Set[Command])
     : List[Command.Edit] =
   {
     val removed = old_cmds.iterator.filter(!new_cmds.contains(_)).toList
     val inserted = new_cmds.iterator.filter(!old_cmds.contains(_)).toList
 
     removed.map(cmd => (old_cmds.prev(cmd), None)) reverse_:::
     inserted.map(cmd => (new_cmds.prev(cmd), Some(cmd)))
   }
 
   private def text_edit(
     resources: Resources,
     syntax: Outer_Syntax,
     get_blob: Document.Node.Name => Option[Document.Blob],
     can_import: Document.Node.Name => Boolean,
     reparse_limit: Int,
     node: Document.Node, edit: Document.Edit_Text): Document.Node =
   {
     /* recover command spans after edits */
     // FIXME somewhat slow
     def recover_spans(
       name: Document.Node.Name,
       perspective: Command.Perspective,
       commands: Linear_Set[Command]): Linear_Set[Command] =
     {
       val is_visible = perspective.commands.toSet
 
       def next_invisible(cmds: Linear_Set[Command], from: Command): Command =
         cmds.iterator(from).dropWhile(cmd => !cmd.is_proper || is_visible(cmd))
           .find(_.is_proper) getOrElse cmds.last
 
       @tailrec def recover(cmds: Linear_Set[Command]): Linear_Set[Command] =
         cmds.find(_.is_unparsed) match {
           case Some(first_unparsed) =>
             val first = next_invisible(cmds.reverse, first_unparsed)
             val last = next_invisible(cmds, first_unparsed)
             recover(
               reparse_spans(resources, syntax, get_blob, can_import, name, cmds, first, last))
           case None => cmds
         }
       recover(commands)
     }
 
     edit match {
       case (_, Document.Node.Blob(blob)) => node.init_blob(blob)
 
       case (name, Document.Node.Edits(text_edits)) =>
         if (name.is_theory) {
           val commands0 = node.commands
           val commands1 = edit_text(text_edits, commands0)
           val commands2 = recover_spans(name, node.perspective.visible, commands1)
           node.update_commands(commands2)
         }
         else node
 
       case (_, Document.Node.Deps(_)) => node
 
       case (name, Document.Node.Perspective(required, text_perspective, overlays)) =>
         val (visible, visible_overlay) = command_perspective(node, text_perspective, overlays)
         val perspective: Document.Node.Perspective_Command =
           Document.Node.Perspective(required, visible_overlay, overlays)
         if (node.same_perspective(text_perspective, perspective)) node
         else {
           /* consolidate unfinished spans */
           val is_visible = visible.commands.toSet
           val commands = node.commands
           val commands1 =
             if (is_visible.isEmpty) commands
             else {
               commands.find(_.is_unfinished) match {
                 case Some(first_unfinished) =>
                   commands.reverse.find(is_visible) match {
                     case Some(last_visible) =>
                       val it = commands.iterator(last_visible)
                       var last = last_visible
                       var i = 0
                       while (i < reparse_limit && it.hasNext) {
                         last = it.next()
                         i += last.length
                       }
                       reparse_spans(resources, syntax, get_blob, can_import,
                         name, commands, first_unfinished, last)
                     case None => commands
                   }
                 case None => commands
               }
             }
           node.update_perspective(text_perspective, perspective).update_commands(commands1)
         }
     }
   }
 
   def parse_change(
       resources: Resources,
       reparse_limit: Int,
       previous: Document.Version,
       doc_blobs: Document.Blobs,
       edits: List[Document.Edit_Text],
       consolidate: List[Document.Node.Name]): Session.Change =
   {
     val (syntax_changed, nodes0, doc_edits0) = header_edits(resources, previous, edits)
 
     def get_blob(name: Document.Node.Name): Option[Document.Blob] =
       doc_blobs.get(name) orElse previous.nodes(name).get_blob
 
     def can_import(name: Document.Node.Name): Boolean =
       resources.session_base.loaded_theory(name) || nodes0(name).has_header
 
     val (doc_edits, version) =
       if (edits.isEmpty) (Nil, Document.Version.make(previous.nodes))
       else {
         val reparse =
           nodes0.iterator.foldLeft(syntax_changed) {
             case (reparse, (name, node)) =>
               if (node.load_commands_changed(doc_blobs) && !reparse.contains(name))
                 name :: reparse
               else reparse
           }
         val reparse_set = reparse.toSet
 
         var nodes = nodes0
         val doc_edits = new mutable.ListBuffer[Document.Edit_Command]; doc_edits ++= doc_edits0
 
         val node_edits =
           (edits ::: reparse.map((_, Document.Node.Edits(Nil)))).groupBy(_._1)
             .asInstanceOf[Map[Document.Node.Name, List[Document.Edit_Text]]]  // FIXME ???
 
         node_edits foreach {
           case (name, edits) =>
             val node = nodes(name)
             val syntax = resources.session_base.node_syntax(nodes, name)
             val commands = node.commands
 
             val node1 =
               if (reparse_set(name) && commands.nonEmpty)
                 node.update_commands(
                   reparse_spans(resources, syntax, get_blob, can_import, name,
                     commands, commands.head, commands.last))
               else node
             val node2 =
               edits.foldLeft(node1)(
                 text_edit(resources, syntax, get_blob, can_import, reparse_limit, _, _))
             val node3 =
               if (reparse_set.contains(name))
                 text_edit(resources, syntax, get_blob, can_import, reparse_limit,
                   node2, (name, node2.edit_perspective))
               else node2
 
             if (!node.same_perspective(node3.text_perspective, node3.perspective))
               doc_edits += (name -> node3.perspective)
 
             doc_edits += (name -> Document.Node.Edits(diff_commands(commands, node3.commands)))
 
             nodes += (name -> node3)
         }
         (doc_edits.toList.filterNot(_._2.is_void), Document.Version.make(nodes))
       }
 
     Session.Change(
       previous, syntax_changed, syntax_changed.nonEmpty, doc_edits, consolidate, version)
   }
 }