diff --git a/src/Pure/General/symbol.scala b/src/Pure/General/symbol.scala
--- a/src/Pure/General/symbol.scala
+++ b/src/Pure/General/symbol.scala
@@ -1,664 +1,668 @@
 /*  Title:      Pure/General/symbol.scala
     Author:     Makarius
 
 Isabelle text symbols.
 */
 
 package isabelle
 
 
 import scala.collection.mutable
 import scala.util.matching.Regex
 import scala.annotation.tailrec
 
 
 object Symbol
 {
   type Symbol = String
 
   // counting Isabelle symbols, starting from 1
   type Offset = Text.Offset
   type Range = Text.Range
 
 
   /* spaces */
 
   val space_char = ' '
   val space = " "
 
   private val static_spaces = space * 4000
 
   def spaces(n: Int): String =
   {
     require(n >= 0)
     if (n < static_spaces.length) static_spaces.substring(0, n)
     else space * n
   }
 
 
   /* ASCII characters */
 
   def is_ascii_printable(c: Char): Boolean = space_char <= c && c <= '~'
 
   def is_ascii_letter(c: Char): Boolean = 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z'
 
   def is_ascii_digit(c: Char): Boolean = '0' <= c && c <= '9'
 
   def is_ascii_hex(c: Char): Boolean =
     '0' <= c && c <= '9' || 'A' <= c && c <= 'F' || 'a' <= c && c <= 'f'
 
   def is_ascii_quasi(c: Char): Boolean = c == '_' || c == '\''
 
   def is_ascii_blank(c: Char): Boolean = " \t\n\u000b\f\r".contains(c)
 
   def is_ascii_line_terminator(c: Char): Boolean = "\r\n".contains(c)
 
   def is_ascii_letdig(c: Char): Boolean =
     is_ascii_letter(c) || is_ascii_digit(c) || is_ascii_quasi(c)
 
   def is_ascii_identifier(s: String): Boolean =
     s.length > 0 && is_ascii_letter(s(0)) && s.forall(is_ascii_letdig)
 
   def ascii(c: Char): Symbol =
   {
     if (c > 127) error("Non-ASCII character: " + quote(c.toString))
     else char_symbols(c.toInt)
   }
 
   def is_ascii(s: Symbol): Boolean = s.length == 1 && s(0) < 128
 
 
   /* symbol matching */
 
   private val symbol_total = new Regex("""(?xs)
     [\ud800-\udbff][\udc00-\udfff] | \r\n | \\ < \^? ([A-Za-z][A-Za-z0-9_']*)? >? | .""")
 
   private def is_plain(c: Char): Boolean =
     !(c == '\r' || c == '\\' || Character.isHighSurrogate(c))
 
   def is_malformed(s: Symbol): Boolean =
     s.length match {
       case 1 =>
         val c = s(0)
         Character.isHighSurrogate(c) || Character.isLowSurrogate(c) || c == '\ufffd'
       case 2 =>
         val c1 = s(0)
         val c2 = s(1)
         !(c1 == '\r' && c2 == '\n' || Character.isSurrogatePair(c1, c2))
       case _ => !s.endsWith(">") || s == "\\<>" || s == "\\<^>"
     }
 
   def is_newline(s: Symbol): Boolean =
     s == "\n" || s == "\r" || s == "\r\n"
 
   class Matcher(text: CharSequence)
   {
     private val matcher = symbol_total.pattern.matcher(text)
     def apply(start: Int, end: Int): Int =
     {
       require(0 <= start && start < end && end <= text.length)
       if (is_plain(text.charAt(start))) 1
       else {
         matcher.region(start, end).lookingAt
         matcher.group.length
       }
     }
   }
 
 
   /* iterator */
 
   private val char_symbols: Array[Symbol] =
     (0 until 256).iterator.map(i => new String(Array(i.toChar))).toArray
 
   def iterator(text: CharSequence): Iterator[Symbol] =
     new Iterator[Symbol]
     {
       private val matcher = new Matcher(text)
       private var i = 0
       def hasNext: Boolean = i < text.length
       def next: Symbol =
       {
         val n = matcher(i, text.length)
         val s =
           if (n == 0) ""
           else if (n == 1) {
             val c = text.charAt(i)
             if (c < char_symbols.length) char_symbols(c)
             else text.subSequence(i, i + n).toString
           }
           else text.subSequence(i, i + n).toString
         i += n
         s
       }
     }
 
   def explode(text: CharSequence): List[Symbol] = iterator(text).toList
 
   def length(text: CharSequence): Int = iterator(text).length
 
   def trim_blanks(text: CharSequence): String =
     Library.trim(is_blank, explode(text)).mkString
 
   def all_blank(str: String): Boolean =
     iterator(str).forall(is_blank)
 
   def trim_blank_lines(text: String): String =
     cat_lines(split_lines(text).dropWhile(all_blank).reverse.dropWhile(all_blank).reverse)
 
 
   /* decoding offsets */
 
   object Index
   {
     private sealed case class Entry(chr: Int, sym: Int)
 
     val empty: Index = new Index(Nil)
 
     def apply(text: CharSequence): Index =
     {
       val matcher = new Matcher(text)
       val buf = new mutable.ListBuffer[Entry]
       var chr = 0
       var sym = 0
       while (chr < text.length) {
         val n = matcher(chr, text.length)
         chr += n
         sym += 1
         if (n > 1) buf += Entry(chr, sym)
       }
       if (buf.isEmpty) empty else new Index(buf.toList)
     }
   }
 
   final class Index private(entries: List[Index.Entry])
   {
     private val hash: Int = entries.hashCode
     private val index: Array[Index.Entry] = entries.toArray
 
     def decode(symbol_offset: Offset): Text.Offset =
     {
       val sym = symbol_offset - 1
       val end = index.length
       @tailrec def bisect(a: Int, b: Int): Int =
       {
         if (a < b) {
           val c = (a + b) / 2
           if (sym < index(c).sym) bisect(a, c)
           else if (c + 1 == end || sym < index(c + 1).sym) c
           else bisect(c + 1, b)
         }
         else -1
       }
       val i = bisect(0, end)
       if (i < 0) sym
       else index(i).chr + sym - index(i).sym
     }
     def decode(symbol_range: Range): Text.Range = symbol_range.map(decode)
 
     override def hashCode: Int = hash
     override def equals(that: Any): Boolean =
       that match {
         case other: Index => index.sameElements(other.index)
         case _ => false
       }
   }
 
 
   /* symbolic text chunks -- without actual text */
 
   object Text_Chunk
   {
     sealed abstract class Name
     case object Default extends Name
     case class Id(id: Document_ID.Generic) extends Name
     case class File(name: String) extends Name
 
     def apply(text: CharSequence): Text_Chunk =
       new Text_Chunk(Text.Range(0, text.length), Index(text))
   }
 
   final class Text_Chunk private(val range: Text.Range, private val index: Index)
   {
     override def hashCode: Int = (range, index).hashCode
     override def equals(that: Any): Boolean =
       that match {
         case other: Text_Chunk =>
           range == other.range &&
           index == other.index
         case _ => false
       }
 
     override def toString: String = "Text_Chunk" + range.toString
 
     def decode(symbol_offset: Offset): Text.Offset = index.decode(symbol_offset)
     def decode(symbol_range: Range): Text.Range = index.decode(symbol_range)
     def incorporate(symbol_range: Range): Option[Text.Range] =
     {
       def in(r: Range): Option[Text.Range] =
         range.try_restrict(decode(r)) match {
           case Some(r1) if !r1.is_singularity => Some(r1)
           case _ => None
         }
      in(symbol_range) orElse in(symbol_range - 1)
     }
   }
 
 
   /* recoding text */
 
   private class Recoder(list: List[(String, String)])
   {
     private val (min, max) =
     {
       var min = '\uffff'
       var max = '\u0000'
       for ((x, _) <- list) {
         val c = x(0)
         if (c < min) min = c
         if (c > max) max = c
       }
       (min, max)
     }
     private val table =
     {
       var tab = Map[String, String]()
       for ((x, y) <- list) {
         tab.get(x) match {
           case None => tab += (x -> y)
           case Some(z) =>
             error("Duplicate symbol mapping of " + quote(x) + " to " + quote(y) + " vs. " + quote(z))
         }
       }
       tab
     }
     def recode(text: String): String =
     {
       val len = text.length
       val matcher = symbol_total.pattern.matcher(text)
       val result = new StringBuilder(len)
       var i = 0
       while (i < len) {
         val c = text(i)
         if (min <= c && c <= max) {
           matcher.region(i, len).lookingAt
           val x = matcher.group
           result.append(table.getOrElse(x, x))
           i = matcher.end
         }
         else { result.append(c); i += 1 }
       }
       result.toString
     }
   }
 
 
 
   /** symbol interpretation **/
 
   val ARGUMENT_CARTOUCHE = "cartouche"
   val ARGUMENT_SPACE_CARTOUCHE = "space_cartouche"
 
   private lazy val symbols =
   {
     val contents =
       for (path <- Path.split(Isabelle_System.getenv("ISABELLE_SYMBOLS")) if path.is_file)
         yield (File.read(path))
     new Interpretation(cat_lines(contents))
   }
 
   private class Interpretation(symbols_spec: String)
   {
     /* read symbols */
 
     private val No_Decl = new Regex("""(?xs) ^\s* (?: \#.* )? $ """)
     private val Key = new Regex("""(?xs) (.+): """)
 
     private def read_decl(decl: String): (Symbol, Properties.T) =
     {
       def err() = error("Bad symbol declaration: " + decl)
 
       def read_props(props: List[String]): Properties.T =
       {
         props match {
           case Nil => Nil
           case _ :: Nil => err()
           case Key(x) :: y :: rest => (x -> y.replace('\u2423', ' ')) :: read_props(rest)
           case _ => err()
         }
       }
       decl.split("\\s+").toList match {
         case sym :: props if sym.length > 1 && !is_malformed(sym) =>
           (sym, read_props(props))
         case _ => err()
       }
     }
 
     private val symbols: List[(Symbol, Properties.T)] =
       (((List.empty[(Symbol, Properties.T)], Set.empty[Symbol]) /:
           split_lines(symbols_spec).reverse)
         { case (res, No_Decl()) => res
           case ((list, known), decl) =>
             val (sym, props) = read_decl(decl)
             if (known(sym)) (list, known)
             else ((sym, props) :: list, known + sym)
         })._1
 
 
     /* basic properties */
 
     val properties: Map[Symbol, Properties.T] = Map(symbols: _*)
 
     val names: Map[Symbol, (String, String)] =
     {
       val Name = new Regex("""\\<\^?([A-Za-z][A-Za-z0-9_']*)>""")
       val Argument = new Properties.String("argument")
       def argument(sym: Symbol, props: Properties.T): String =
         props match {
           case Argument(arg) =>
             if (arg == ARGUMENT_CARTOUCHE || arg == ARGUMENT_SPACE_CARTOUCHE) arg
             else error("Bad argument: " + quote(arg) + " for symbol " + quote(sym))
           case _ => ""
         }
       Map((for ((sym @ Name(a), props) <- symbols) yield sym -> (a, argument(sym, props))): _*)
     }
 
     val groups: List[(String, List[Symbol])] =
       symbols.flatMap({ case (sym, props) =>
         val gs = for (("group", g) <- props) yield g
         if (gs.isEmpty) List(sym -> "unsorted") else gs.map(sym -> _)
       }).groupBy(_._2).toList.map({ case (group, list) => (group, list.map(_._1)) })
         .sortBy(_._1)
 
     val abbrevs: Multi_Map[Symbol, String] =
       Multi_Map((
         for {
           (sym, props) <- symbols
           ("abbrev", a) <- props.reverse
         } yield sym -> a): _*)
 
     val codes: List[(Symbol, Int)] =
     {
       val Code = new Properties.String("code")
       for {
         (sym, props) <- symbols
         code <-
           props match {
             case Code(s) =>
               try { Some(Integer.decode(s).intValue) }
               catch { case _: NumberFormatException => error("Bad code for symbol " + sym) }
             case _ => None
           }
       } yield {
         if (code < 128) error("Illegal ASCII code for symbol " + sym)
         else (sym, code)
       }
     }
 
 
     /* recoding */
 
     private val (decoder, encoder) =
     {
       val mapping =
         for ((sym, code) <- codes) yield (sym, new String(Character.toChars(code)))
       (new Recoder(mapping), new Recoder(for ((x, y) <- mapping) yield (y, x)))
     }
 
     def decode(text: String): String = decoder.recode(text)
     def encode(text: String): String = encoder.recode(text)
 
     private def recode_set(elems: String*): Set[String] =
     {
       val content = elems.toList
       Set((content ::: content.map(decode)): _*)
     }
 
     private def recode_map[A](elems: (String, A)*): Map[String, A] =
     {
       val content = elems.toList
       Map((content ::: content.map({ case (sym, a) => (decode(sym), a) })): _*)
     }
 
 
     /* user fonts */
 
     private val Font = new Properties.String("font")
     val fonts: Map[Symbol, String] =
       recode_map((for ((sym, Font(font)) <- symbols) yield sym -> font): _*)
 
     val font_names: List[String] = Set(fonts.toList.map(_._2): _*).toList
     val font_index: Map[String, Int] = Map((font_names zip font_names.indices.toList): _*)
 
 
     /* classification */
 
     val letters: Set[String] = recode_set(
       "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
       "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z",
       "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m",
       "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z",
 
       "\\<A>", "\\<B>", "\\<C>", "\\<D>", "\\<E>", "\\<F>", "\\<G>",
       "\\<H>", "\\<I>", "\\<J>", "\\<K>", "\\<L>", "\\<M>", "\\<N>",
       "\\<O>", "\\<P>", "\\<Q>", "\\<R>", "\\<S>", "\\<T>", "\\<U>",
       "\\<V>", "\\<W>", "\\<X>", "\\<Y>", "\\<Z>", "\\<a>", "\\<b>",
       "\\<c>", "\\<d>", "\\<e>", "\\<f>", "\\<g>", "\\<h>", "\\<i>",
       "\\<j>", "\\<k>", "\\<l>", "\\<m>", "\\<n>", "\\<o>", "\\<p>",
       "\\<q>", "\\<r>", "\\<s>", "\\<t>", "\\<u>", "\\<v>", "\\<w>",
       "\\<x>", "\\<y>", "\\<z>",
 
       "\\<AA>", "\\<BB>", "\\<CC>", "\\<DD>", "\\<EE>", "\\<FF>",
       "\\<GG>", "\\<HH>", "\\<II>", "\\<JJ>", "\\<KK>", "\\<LL>",
       "\\<MM>", "\\<NN>", "\\<OO>", "\\<PP>", "\\<QQ>", "\\<RR>",
       "\\<SS>", "\\<TT>", "\\<UU>", "\\<VV>", "\\<WW>", "\\<XX>",
       "\\<YY>", "\\<ZZ>", "\\<aa>", "\\<bb>", "\\<cc>", "\\<dd>",
       "\\<ee>", "\\<ff>", "\\<gg>", "\\<hh>", "\\<ii>", "\\<jj>",
       "\\<kk>", "\\<ll>", "\\<mm>", "\\<nn>", "\\<oo>", "\\<pp>",
       "\\<qq>", "\\<rr>", "\\<ss>", "\\<tt>", "\\<uu>", "\\<vv>",
       "\\<ww>", "\\<xx>", "\\<yy>", "\\<zz>",
 
       "\\<alpha>", "\\<beta>", "\\<gamma>", "\\<delta>", "\\<epsilon>",
       "\\<zeta>", "\\<eta>", "\\<theta>", "\\<iota>", "\\<kappa>",
       "\\<mu>", "\\<nu>", "\\<xi>", "\\<pi>", "\\<rho>", "\\<sigma>",
       "\\<tau>", "\\<upsilon>", "\\<phi>", "\\<chi>", "\\<psi>",
       "\\<omega>", "\\<Gamma>", "\\<Delta>", "\\<Theta>", "\\<Lambda>",
       "\\<Xi>", "\\<Pi>", "\\<Sigma>", "\\<Upsilon>", "\\<Phi>",
       "\\<Psi>", "\\<Omega>")
 
     val blanks: Set[String] = recode_set(space, "\t", "\n", "\u000B", "\f", "\r", "\r\n")
 
     val sym_chars =
       Set("!", "#", "$", "%", "&", "*", "+", "-", "/", "<", "=", ">", "?", "@", "^", "_", "|", "~")
 
     val symbolic: Set[String] = recode_set((for {(sym, _) <- symbols; if raw_symbolic(sym)} yield sym): _*)
 
 
     /* misc symbols */
 
     val newline_decoded = decode(newline)
     val comment_decoded = decode(comment)
     val cancel_decoded = decode(cancel)
     val latex_decoded = decode(latex)
     val marker_decoded = decode(marker)
     val open_decoded = decode(open)
     val close_decoded = decode(close)
 
 
     /* control symbols */
 
     val control_decoded: Set[Symbol] =
       Set((for ((sym, _) <- symbols if sym.startsWith("\\<^")) yield decode(sym)): _*)
 
     val sub_decoded = decode(sub)
     val sup_decoded = decode(sup)
     val bold_decoded = decode(bold)
     val emph_decoded = decode(emph)
     val bsub_decoded = decode(bsub)
     val esub_decoded = decode(esub)
     val bsup_decoded = decode(bsup)
     val esup_decoded = decode(esup)
   }
 
 
   /* tables */
 
   def properties: Map[Symbol, Properties.T] = symbols.properties
   def names: Map[Symbol, (String, String)] = symbols.names
   def groups: List[(String, List[Symbol])] = symbols.groups
   def abbrevs: Multi_Map[Symbol, String] = symbols.abbrevs
   def codes: List[(Symbol, Int)] = symbols.codes
   def groups_code: List[(String, List[Symbol])] =
   {
     val has_code = codes.iterator.map(_._1).toSet
     groups.flatMap({ case (group, symbols) =>
       val symbols1 = symbols.filter(has_code)
       if (symbols1.isEmpty) None else Some((group, symbols1))
     })
   }
 
   lazy val is_code: Int => Boolean = codes.map(_._2).toSet
   def decode(text: String): String = symbols.decode(text)
   def encode(text: String): String = symbols.encode(text)
 
-  def decode_yxml(text: String): XML.Body = YXML.parse_body(decode(text))
-  def decode_yxml_failsafe(text: String): XML.Body = YXML.parse_body_failsafe(decode(text))
+  def decode_yxml(text: String, cache: XML.Cache = XML.Cache.none): XML.Body =
+    YXML.parse_body(decode(text), cache = cache)
+
+  def decode_yxml_failsafe(text: String, cache: XML.Cache = XML.Cache.none): XML.Body =
+    YXML.parse_body_failsafe(decode(text), cache = cache)
+
   def encode_yxml(body: XML.Body): String = encode(YXML.string_of_body(body))
 
   def decode_strict(text: String): String =
   {
     val decoded = decode(text)
     if (encode(decoded) == text) decoded
     else {
       val bad = new mutable.ListBuffer[Symbol]
       for (s <- iterator(text) if encode(decode(s)) != s && !bad.contains(s))
         bad += s
       error("Bad Unicode symbols in text: " + commas_quote(bad))
     }
   }
 
   def output(unicode_symbols: Boolean, text: String): String =
     if (unicode_symbols) Symbol.decode(text) else Symbol.encode(text)
 
   def fonts: Map[Symbol, String] = symbols.fonts
   def font_names: List[String] = symbols.font_names
   def font_index: Map[String, Int] = symbols.font_index
   def lookup_font(sym: Symbol): Option[Int] = symbols.fonts.get(sym).map(font_index(_))
 
 
   /* classification */
 
   def is_letter(sym: Symbol): Boolean = symbols.letters.contains(sym)
   def is_digit(sym: Symbol): Boolean = sym.length == 1 && '0' <= sym(0) && sym(0) <= '9'
   def is_quasi(sym: Symbol): Boolean = sym == "_" || sym == "'"
   def is_letdig(sym: Symbol): Boolean = is_letter(sym) || is_digit(sym) || is_quasi(sym)
   def is_blank(sym: Symbol): Boolean = symbols.blanks.contains(sym)
 
 
   /* symbolic newline */
 
   val newline: Symbol = "\\<newline>"
   def newline_decoded: Symbol = symbols.newline_decoded
 
   def print_newlines(str: String): String =
     if (str.contains('\n'))
       (for (s <- iterator(str)) yield { if (s == "\n") newline_decoded else s }).mkString
     else str
 
 
   /* formal comments */
 
   val comment: Symbol = "\\<comment>"
   val cancel: Symbol = "\\<^cancel>"
   val latex: Symbol = "\\<^latex>"
   val marker: Symbol = "\\<^marker>"
 
   def comment_decoded: Symbol = symbols.comment_decoded
   def cancel_decoded: Symbol = symbols.cancel_decoded
   def latex_decoded: Symbol = symbols.latex_decoded
   def marker_decoded: Symbol = symbols.marker_decoded
 
 
   /* cartouches */
 
   val open: Symbol = "\\<open>"
   val close: Symbol = "\\<close>"
 
   def open_decoded: Symbol = symbols.open_decoded
   def close_decoded: Symbol = symbols.close_decoded
 
   def is_open(sym: Symbol): Boolean = sym == open_decoded || sym == open
   def is_close(sym: Symbol): Boolean = sym == close_decoded || sym == close
 
   def cartouche(s: String): String = open + s + close
   def cartouche_decoded(s: String): String = open_decoded + s + close_decoded
 
 
   /* symbols for symbolic identifiers */
 
   private def raw_symbolic(sym: Symbol): Boolean =
     sym.startsWith("\\<") && sym.endsWith(">") && !sym.startsWith("\\<^")
 
   def is_symbolic(sym: Symbol): Boolean =
     !is_open(sym) && !is_close(sym) && (raw_symbolic(sym) || symbols.symbolic.contains(sym))
 
   def is_symbolic_char(sym: Symbol): Boolean = symbols.sym_chars.contains(sym)
 
 
   /* control symbols */
 
   val control_prefix = "\\<^"
   val control_suffix = ">"
 
   def control_name(sym: Symbol): Option[String] =
     if (is_control_encoded(sym))
       Some(sym.substring(control_prefix.length, sym.length - control_suffix.length))
     else None
 
   def is_control_encoded(sym: Symbol): Boolean =
     sym.startsWith(control_prefix) && sym.endsWith(control_suffix)
 
   def is_control(sym: Symbol): Boolean =
     is_control_encoded(sym) || symbols.control_decoded.contains(sym)
 
   def is_controllable(sym: Symbol): Boolean =
     !is_blank(sym) && !is_control(sym) && !is_open(sym) && !is_close(sym) &&
     !is_malformed(sym) && sym != "\""
 
   val sub = "\\<^sub>"
   val sup = "\\<^sup>"
   val bold = "\\<^bold>"
   val emph = "\\<^emph>"
   val bsub = "\\<^bsub>"
   val esub = "\\<^esub>"
   val bsup = "\\<^bsup>"
   val esup = "\\<^esup>"
 
   def sub_decoded: Symbol = symbols.sub_decoded
   def sup_decoded: Symbol = symbols.sup_decoded
   def bold_decoded: Symbol = symbols.bold_decoded
   def emph_decoded: Symbol = symbols.emph_decoded
   def bsub_decoded: Symbol = symbols.bsub_decoded
   def esub_decoded: Symbol = symbols.esub_decoded
   def bsup_decoded: Symbol = symbols.bsup_decoded
   def esup_decoded: Symbol = symbols.esup_decoded
 
 
   /* metric */
 
   def is_printable(sym: Symbol): Boolean =
     if (is_ascii(sym)) is_ascii_printable(sym(0))
     else !is_control(sym)
 
   object Metric extends Pretty.Metric
   {
     val unit = 1.0
     def apply(str: String): Double =
       (for (s <- iterator(str)) yield {
         val sym = encode(s)
         if (sym.startsWith("\\<long") || sym.startsWith("\\<Long")) 2
         else if (is_printable(sym)) 1
         else 0
       }).sum
   }
 }
diff --git a/src/Pure/PIDE/xml.scala b/src/Pure/PIDE/xml.scala
--- a/src/Pure/PIDE/xml.scala
+++ b/src/Pure/PIDE/xml.scala
@@ -1,429 +1,433 @@
 /*  Title:      Pure/PIDE/xml.scala
     Author:     Makarius
 
 Untyped XML trees and basic data representation.
 */
 
 package isabelle
 
 
 object XML
 {
   /** XML trees **/
 
   /* datatype representation */
 
   type Attribute = Properties.Entry
   type Attributes = Properties.T
 
   sealed abstract class Tree { override def toString: String = string_of_tree(this) }
   type Body = List[Tree]
   case class Elem(markup: Markup, body: Body) extends Tree
   {
     def name: String = markup.name
 
     def update_attributes(more_attributes: Attributes): Elem =
       if (more_attributes.isEmpty) this
       else Elem(markup.update_properties(more_attributes), body)
 
     def + (att: Attribute): Elem = Elem(markup + att, body)
   }
   case class Text(content: String) extends Tree
 
   def elem(markup: Markup): XML.Elem = XML.Elem(markup, Nil)
   def elem(name: String, body: Body): XML.Elem = XML.Elem(Markup(name, Nil), body)
   def elem(name: String): XML.Elem = XML.Elem(Markup(name, Nil), Nil)
 
   val no_text: Text = Text("")
   val newline: Text = Text("\n")
 
 
   /* name space */
 
   object Namespace
   {
     def apply(prefix: String, target: String): Namespace =
       new Namespace(prefix, target)
   }
 
   final class Namespace private(prefix: String, target: String)
   {
     def apply(name: String): String = prefix + ":" + name
     val attribute: XML.Attribute = ("xmlns:" + prefix, target)
 
     override def toString: String = attribute.toString
   }
 
 
   /* wrapped elements */
 
   val XML_ELEM = "xml_elem"
   val XML_NAME = "xml_name"
   val XML_BODY = "xml_body"
 
   object Wrapped_Elem
   {
     def apply(markup: Markup, body1: Body, body2: Body): XML.Elem =
       XML.Elem(Markup(XML_ELEM, (XML_NAME, markup.name) :: markup.properties),
         XML.Elem(Markup(XML_BODY, Nil), body1) :: body2)
 
     def unapply(tree: Tree): Option[(Markup, Body, Body)] =
       tree match {
         case
           XML.Elem(Markup(XML_ELEM, (XML_NAME, name) :: props),
             XML.Elem(Markup(XML_BODY, Nil), body1) :: body2) =>
           Some(Markup(name, props), body1, body2)
         case _ => None
       }
   }
 
   object Root_Elem
   {
     def apply(body: Body): XML.Elem = XML.Elem(Markup(XML_ELEM, Nil), body)
     def unapply(tree: Tree): Option[Body] =
       tree match {
         case XML.Elem(Markup(XML_ELEM, Nil), body) => Some(body)
         case _ => None
       }
   }
 
 
   /* traverse text */
 
   def traverse_text[A](body: Body)(a: A)(op: (A, String) => A): A =
   {
     def traverse(x: A, t: Tree): A =
       t match {
         case XML.Wrapped_Elem(_, _, ts) => (x /: ts)(traverse)
         case XML.Elem(_, ts) => (x /: ts)(traverse)
         case XML.Text(s) => op(x, s)
       }
     (a /: body)(traverse)
   }
 
   def text_length(body: Body): Int = traverse_text(body)(0) { case (n, s) => n + s.length }
 
 
   /* text content */
 
   def content(body: Body): String =
   {
     val text = new StringBuilder(text_length(body))
     traverse_text(body)(()) { case (_, s) => text.append(s) }
     text.toString
   }
 
   def content(tree: Tree): String = content(List(tree))
 
 
 
   /** string representation **/
 
   val header: String = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n"
 
   def output_char(c: Char, s: StringBuilder)
   {
     c match {
       case '<' => s ++= "&lt;"
       case '>' => s ++= "&gt;"
       case '&' => s ++= "&amp;"
       case '"' => s ++= "&quot;"
       case '\'' => s ++= "&apos;"
       case _ => s += c
     }
   }
 
   def output_string(str: String, s: StringBuilder)
   {
     if (str == null) s ++= str
     else str.iterator.foreach(c => output_char(c, s))
   }
 
   def string_of_body(body: Body): String =
   {
     val s = new StringBuilder
 
     def text(txt: String) { output_string(txt, s) }
     def elem(markup: Markup)
     {
       s ++= markup.name
       for ((a, b) <- markup.properties) {
         s += ' '; s ++= a; s += '='; s += '"'; text(b); s += '"'
       }
     }
     def tree(t: Tree): Unit =
       t match {
         case XML.Elem(markup, Nil) =>
           s += '<'; elem(markup); s ++= "/>"
         case XML.Elem(markup, ts) =>
           s += '<'; elem(markup); s += '>'
           ts.foreach(tree)
           s ++= "</"; s ++= markup.name; s += '>'
         case XML.Text(txt) => text(txt)
       }
     body.foreach(tree)
     s.toString
   }
 
   def string_of_tree(tree: XML.Tree): String = string_of_body(List(tree))
 
 
 
   /** cache **/
 
   object Cache
   {
     def make(
         max_string: Int = isabelle.Cache.default_max_string,
         initial_size: Int = isabelle.Cache.default_initial_size): Cache =
       new Cache(max_string, initial_size)
 
     val none: Cache = make(max_string = 0)
   }
 
   class Cache private[XML](max_string: Int, initial_size: Int)
     extends isabelle.Cache(max_string, initial_size)
   {
     protected def cache_props(x: Properties.T): Properties.T =
     {
       if (x.isEmpty) x
       else
         lookup(x) match {
           case Some(y) => y
           case None => store(x.map(p => (Library.isolate_substring(p._1).intern, cache_string(p._2))))
         }
     }
 
     protected def cache_markup(x: Markup): Markup =
     {
       lookup(x) match {
         case Some(y) => y
         case None =>
           x match {
             case Markup(name, props) =>
               store(Markup(cache_string(name), cache_props(props)))
           }
       }
     }
 
     protected def cache_tree(x: XML.Tree): XML.Tree =
     {
       lookup(x) match {
         case Some(y) => y
         case None =>
           x match {
             case XML.Elem(markup, body) =>
               store(XML.Elem(cache_markup(markup), cache_body(body)))
             case XML.Text(text) => store(XML.Text(cache_string(text)))
           }
       }
     }
 
     protected def cache_body(x: XML.Body): XML.Body =
     {
       if (x.isEmpty) x
       else
         lookup(x) match {
           case Some(y) => y
           case None => x.map(cache_tree)
         }
     }
 
+    // support hash-consing
+    def tree0(x: XML.Tree): XML.Tree =
+      if (no_cache) x else synchronized { lookup(x) getOrElse store(x) }
+
     // main methods
     def props(x: Properties.T): Properties.T =
       if (no_cache) x else synchronized { cache_props(x) }
     def markup(x: Markup): Markup =
       if (no_cache) x else synchronized { cache_markup(x) }
     def tree(x: XML.Tree): XML.Tree =
       if (no_cache) x else synchronized { cache_tree(x) }
     def body(x: XML.Body): XML.Body =
       if (no_cache) x else synchronized { cache_body(x) }
     def elem(x: XML.Elem): XML.Elem =
       if (no_cache) x else synchronized { cache_tree(x).asInstanceOf[XML.Elem] }
   }
 
 
 
   /** XML as data representation language **/
 
   abstract class Error(s: String) extends Exception(s)
   class XML_Atom(s: String) extends Error(s)
   class XML_Body(body: XML.Body) extends Error("")
 
   object Encode
   {
     type T[A] = A => XML.Body
     type V[A] = PartialFunction[A, (List[String], XML.Body)]
     type P[A] = PartialFunction[A, List[String]]
 
 
     /* atomic values */
 
     def long_atom(i: Long): String = Library.signed_string_of_long(i)
 
     def int_atom(i: Int): String = Library.signed_string_of_int(i)
 
     def bool_atom(b: Boolean): String = if (b) "1" else "0"
 
     def unit_atom(u: Unit) = ""
 
 
     /* structural nodes */
 
     private def node(ts: XML.Body): XML.Tree = XML.Elem(Markup(":", Nil), ts)
 
     private def vector(xs: List[String]): XML.Attributes =
       xs.zipWithIndex.map({ case (x, i) => (int_atom(i), x) })
 
     private def tagged(tag: Int, data: (List[String], XML.Body)): XML.Tree =
       XML.Elem(Markup(int_atom(tag), vector(data._1)), data._2)
 
 
     /* representation of standard types */
 
     val tree: T[XML.Tree] = (t => List(t))
 
     val properties: T[Properties.T] =
       (props => List(XML.Elem(Markup(":", props), Nil)))
 
     val string: T[String] = (s => if (s.isEmpty) Nil else List(XML.Text(s)))
 
     val long: T[Long] = (x => string(long_atom(x)))
 
     val int: T[Int] = (x => string(int_atom(x)))
 
     val bool: T[Boolean] = (x => string(bool_atom(x)))
 
     val unit: T[Unit] = (x => string(unit_atom(x)))
 
     def pair[A, B](f: T[A], g: T[B]): T[(A, B)] =
       (x => List(node(f(x._1)), node(g(x._2))))
 
     def triple[A, B, C](f: T[A], g: T[B], h: T[C]): T[(A, B, C)] =
       (x => List(node(f(x._1)), node(g(x._2)), node(h(x._3))))
 
     def list[A](f: T[A]): T[List[A]] =
       (xs => xs.map((x: A) => node(f(x))))
 
     def option[A](f: T[A]): T[Option[A]] =
     {
       case None => Nil
       case Some(x) => List(node(f(x)))
     }
 
     def variant[A](fs: List[V[A]]): T[A] =
     {
       case x =>
         val (f, tag) = fs.iterator.zipWithIndex.find(p => p._1.isDefinedAt(x)).get
         List(tagged(tag, f(x)))
     }
   }
 
   object Decode
   {
     type T[A] = XML.Body => A
     type V[A] = (List[String], XML.Body) => A
     type P[A] = PartialFunction[List[String], A]
 
 
     /* atomic values */
 
     def long_atom(s: String): Long =
       try { java.lang.Long.parseLong(s) }
       catch { case e: NumberFormatException => throw new XML_Atom(s) }
 
     def int_atom(s: String): Int =
       try { Integer.parseInt(s) }
       catch { case e: NumberFormatException => throw new XML_Atom(s) }
 
     def bool_atom(s: String): Boolean =
       if (s == "1") true
       else if (s == "0") false
       else throw new XML_Atom(s)
 
     def unit_atom(s: String): Unit =
       if (s == "") () else throw new XML_Atom(s)
 
 
     /* structural nodes */
 
     private def node(t: XML.Tree): XML.Body =
       t match {
         case XML.Elem(Markup(":", Nil), ts) => ts
         case _ => throw new XML_Body(List(t))
       }
 
     private def vector(atts: XML.Attributes): List[String] =
       atts.iterator.zipWithIndex.map(
         { case ((a, x), i) => if (int_atom(a) == i) x else throw new XML_Atom(a) }).toList
 
     private def tagged(t: XML.Tree): (Int, (List[String], XML.Body)) =
       t match {
         case XML.Elem(Markup(name, atts), ts) => (int_atom(name), (vector(atts), ts))
         case _ => throw new XML_Body(List(t))
       }
 
 
     /* representation of standard types */
 
     val tree: T[XML.Tree] =
     {
       case List(t) => t
       case ts => throw new XML_Body(ts)
     }
 
     val properties: T[Properties.T] =
     {
       case List(XML.Elem(Markup(":", props), Nil)) => props
       case ts => throw new XML_Body(ts)
     }
 
     val string: T[String] =
     {
       case Nil => ""
       case List(XML.Text(s)) => s
       case ts => throw new XML_Body(ts)
     }
 
     val long: T[Long] = (x => long_atom(string(x)))
 
     val int: T[Int] = (x => int_atom(string(x)))
 
     val bool: T[Boolean] = (x => bool_atom(string(x)))
 
     val unit: T[Unit] = (x => unit_atom(string(x)))
 
     def pair[A, B](f: T[A], g: T[B]): T[(A, B)] =
     {
       case List(t1, t2) => (f(node(t1)), g(node(t2)))
       case ts => throw new XML_Body(ts)
     }
 
     def triple[A, B, C](f: T[A], g: T[B], h: T[C]): T[(A, B, C)] =
     {
       case List(t1, t2, t3) => (f(node(t1)), g(node(t2)), h(node(t3)))
       case ts => throw new XML_Body(ts)
     }
 
     def list[A](f: T[A]): T[List[A]] =
       (ts => ts.map(t => f(node(t))))
 
     def option[A](f: T[A]): T[Option[A]] =
     {
       case Nil => None
       case List(t) => Some(f(node(t)))
       case ts => throw new XML_Body(ts)
     }
 
     def variant[A](fs: List[V[A]]): T[A] =
     {
       case List(t) =>
         val (tag, (xs, ts)) = tagged(t)
         val f =
           try { fs(tag) }
           catch { case _: IndexOutOfBoundsException => throw new XML_Body(List(t)) }
         f(xs, ts)
       case ts => throw new XML_Body(ts)
     }
   }
 }
diff --git a/src/Pure/PIDE/yxml.scala b/src/Pure/PIDE/yxml.scala
--- a/src/Pure/PIDE/yxml.scala
+++ b/src/Pure/PIDE/yxml.scala
@@ -1,156 +1,156 @@
 /*  Title:      Pure/PIDE/yxml.scala
     Author:     Makarius
 
 Efficient text representation of XML trees.  Suitable for direct
 inlining into plain text.
 */
 
 package isabelle
 
 
 import scala.collection.mutable
 
 
 object YXML
 {
   /* chunk markers */
 
   val X = '\u0005'
   val Y = '\u0006'
 
   val is_X: Char => Boolean = _ == X
   val is_Y: Char => Boolean = _ == Y
 
   val X_string: String = X.toString
   val Y_string: String = Y.toString
   val XY_string: String = X_string + Y_string
   val XYX_string: String = XY_string + X_string
 
   def detect(s: String): Boolean = s.exists(c => c == X || c == Y)
   def detect_elem(s: String): Boolean = s.startsWith(XY_string)
 
 
   /* string representation */
 
   def traversal(string: String => Unit, body: XML.Body)
   {
     def tree(t: XML.Tree): Unit =
       t match {
         case XML.Elem(Markup(name, atts), ts) =>
           string(XY_string)
           string(name)
           for ((a, x) <- atts) { string(Y_string); string(a); string("="); string(x) }
           string(X_string)
           ts.foreach(tree)
           string(XYX_string)
         case XML.Text(text) => string(text)
       }
     body.foreach(tree)
   }
 
   def string_of_body(body: XML.Body): String =
   {
     val s = new StringBuilder
     traversal(str => s ++= str, body)
     s.toString
   }
 
   def string_of_tree(tree: XML.Tree): String = string_of_body(List(tree))
 
 
   /* parsing */
 
   private def err(msg: String) = error("Malformed YXML: " + msg)
   private def err_attribute() = err("bad attribute")
   private def err_element() = err("bad element")
   private def err_unbalanced(name: String) =
     if (name == "") err("unbalanced element")
     else err("unbalanced element " + quote(name))
 
   private def parse_attrib(source: CharSequence): (String, String) =
   {
     val s = source.toString
     val i = s.indexOf('=')
     if (i <= 0) err_attribute()
     (s.substring(0, i), s.substring(i + 1))
   }
 
 
-  def parse_body(source: CharSequence): XML.Body =
+  def parse_body(source: CharSequence, cache: XML.Cache = XML.Cache.none): XML.Body =
   {
     /* stack operations */
 
     def buffer(): mutable.ListBuffer[XML.Tree] = new mutable.ListBuffer[XML.Tree]
     var stack: List[(Markup, mutable.ListBuffer[XML.Tree])] = List((Markup.Empty, buffer()))
 
     def add(x: XML.Tree)
     {
       (stack: @unchecked) match { case (_, body) :: _ => body += x }
     }
 
     def push(name: String, atts: XML.Attributes)
     {
       if (name == "") err_element()
-      else stack = (Markup(name, atts), buffer()) :: stack
+      else stack = (cache.markup(Markup(name, atts)), buffer()) :: stack
     }
 
     def pop()
     {
       (stack: @unchecked) match {
         case (Markup.Empty, _) :: _ => err_unbalanced("")
         case (markup, body) :: pending =>
           stack = pending
-          add(XML.Elem(markup, body.toList))
+          add(cache.tree0(XML.Elem(markup, body.toList)))
       }
     }
 
 
     /* parse chunks */
 
     for (chunk <- Library.separated_chunks(is_X, source) if chunk.length != 0) {
       if (chunk.length == 1 && chunk.charAt(0) == Y) pop()
       else {
         Library.separated_chunks(is_Y, chunk).toList match {
           case ch :: name :: atts if ch.length == 0 =>
             push(name.toString, atts.map(parse_attrib))
-          case txts => for (txt <- txts) add(XML.Text(txt.toString))
+          case txts => for (txt <- txts) add(cache.tree0(XML.Text(cache.string(txt.toString))))
         }
       }
     }
     (stack: @unchecked) match {
       case List((Markup.Empty, body)) => body.toList
       case (Markup(name, _), _) :: _ => err_unbalanced(name)
     }
   }
 
-  def parse(source: CharSequence): XML.Tree =
-    parse_body(source) match {
+  def parse(source: CharSequence, cache: XML.Cache = XML.Cache.none): XML.Tree =
+    parse_body(source, cache = cache) match {
       case List(result) => result
       case Nil => XML.no_text
       case _ => err("multiple XML trees")
     }
 
-  def parse_elem(source: CharSequence): XML.Tree =
-    parse_body(source) match {
+  def parse_elem(source: CharSequence, cache: XML.Cache = XML.Cache.none): XML.Tree =
+    parse_body(source, cache = cache) match {
       case List(elem: XML.Elem) => elem
       case _ => err("single XML element expected")
     }
 
 
   /* failsafe parsing */
 
   private def markup_broken(source: CharSequence) =
     XML.Elem(Markup.Broken, List(XML.Text(source.toString)))
 
-  def parse_body_failsafe(source: CharSequence): XML.Body =
+  def parse_body_failsafe(source: CharSequence, cache: XML.Cache = XML.Cache.none): XML.Body =
   {
-    try { parse_body(source) }
+    try { parse_body(source, cache = cache) }
     catch { case ERROR(_) => List(markup_broken(source)) }
   }
 
-  def parse_failsafe(source: CharSequence): XML.Tree =
+  def parse_failsafe(source: CharSequence, cache: XML.Cache = XML.Cache.none): XML.Tree =
   {
-    try { parse(source) }
+    try { parse(source, cache = cache) }
     catch { case ERROR(_) => markup_broken(source) }
   }
 }