diff --git a/src/Pure/General/path.scala b/src/Pure/General/path.scala
--- a/src/Pure/General/path.scala
+++ b/src/Pure/General/path.scala
@@ -1,260 +1,263 @@
 /*  Title:      Pure/General/path.scala
     Author:     Makarius
 
 Algebra of file-system paths: basic POSIX notation, extended by named
 roots (e.g. //foo) and variables (e.g. $BAR).
 */
 
 package isabelle
 
 
 import java.io.{File => JFile}
 
 import scala.util.matching.Regex
 
 
 object Path
 {
   /* path elements */
 
   sealed abstract class Elem
   private case class Root(name: String) extends Elem
   private case class Basic(name: String) extends Elem
   private case class Variable(name: String) extends Elem
   private case object Parent extends Elem
 
   private def err_elem(msg: String, s: String): Nothing =
     error(msg + " path element " + quote(s))
 
   private val illegal_elem = Set("", "~", "~~", ".", "..")
   private val illegal_char = "/\\$:\"'<>|?*"
 
   private def check_elem(s: String): String =
     if (illegal_elem.contains(s)) err_elem("Illegal", s)
     else {
       for (c <- s) {
         if (c.toInt < 32)
           err_elem("Illegal control character " + c.toInt + " in", s)
         if (illegal_char.contains(c))
           err_elem("Illegal character " + quote(c.toString) + " in", s)
       }
       s
     }
 
   private def root_elem(s: String): Elem = Root(check_elem(s))
   private def basic_elem(s: String): Elem = Basic(check_elem(s))
   private def variable_elem(s: String): Elem = Variable(check_elem(s))
 
   private def apply_elem(y: Elem, xs: List[Elem]): List[Elem] =
     (y, xs) match {
       case (Root(_), _) => List(y)
       case (Parent, Root(_) :: _) => xs
       case (Parent, Basic(_) :: rest) => rest
       case _ => y :: xs
     }
 
   private def norm_elems(elems: List[Elem]): List[Elem] =
     (elems :\ (Nil: List[Elem]))(apply_elem)
 
   private def implode_elem(elem: Elem, short: Boolean): String =
     elem match {
       case Root("") => ""
       case Root(s) => "//" + s
       case Basic(s) => s
       case Variable("USER_HOME") if short => "~"
       case Variable("ISABELLE_HOME") if short => "~~"
       case Variable(s) => "$" + s
       case Parent => ".."
     }
 
 
   /* path constructors */
 
   val current: Path = new Path(Nil)
   val root: Path = new Path(List(Root("")))
   def named_root(s: String): Path = new Path(List(root_elem(s)))
   def make(elems: List[String]): Path = new Path(elems.reverse.map(basic_elem))
   def basic(s: String): Path = new Path(List(basic_elem(s)))
   def variable(s: String): Path = new Path(List(variable_elem(s)))
   val parent: Path = new Path(List(Parent))
 
 
   /* explode */
 
   def explode(str: String): Path =
   {
     def explode_elem(s: String): Elem =
       try {
         if (s == "..") Parent
         else if (s == "~") Variable("USER_HOME")
         else if (s == "~~") Variable("ISABELLE_HOME")
         else if (s.startsWith("$")) variable_elem(s.substring(1))
         else basic_elem(s)
       }
       catch { case ERROR(msg) => cat_error(msg, "The error(s) above occurred in " + quote(str)) }
   
     val ss = space_explode('/', str)
     val r = ss.takeWhile(_.isEmpty).length
     val es = ss.dropWhile(_.isEmpty)
     val (roots, raw_elems) =
       if (r == 0) (Nil, es)
       else if (r == 1) (List(Root("")), es)
       else if (es.isEmpty) (List(Root("")), Nil)
       else (List(root_elem(es.head)), es.tail)
     val elems = raw_elems.filterNot(s => s.isEmpty || s == ".").map(explode_elem)
 
     new Path(norm_elems(elems reverse_::: roots))
   }
 
   def is_wellformed(str: String): Boolean =
     try { explode(str); true } catch { case ERROR(_) => false }
 
   def is_valid(str: String): Boolean =
     try { explode(str).expand; true } catch { case ERROR(_) => false }
 
   def split(str: String): List[Path] =
     space_explode(':', str).filterNot(_.isEmpty).map(explode)
 
 
   /* encode */
 
   val encode: XML.Encode.T[Path] = (path => XML.Encode.string(path.implode))
 
 
   /* reserved names */
 
   private val reserved_windows: Set[String] =
     Set("CON", "PRN", "AUX", "NUL",
       "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8", "COM9",
       "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9")
 
   def is_reserved(name: String): Boolean =
     Long_Name.explode(name).exists(a => reserved_windows.contains(Word.uppercase(a)))
 
 
   /* case-insensitive names */
 
   def check_case_insensitive(paths: List[Path])
   {
     val table =
       (Multi_Map.empty[String, String] /: paths)({ case (tab, path) =>
         val name = path.expand.implode
         tab.insert(Word.lowercase(name), name)
       })
     val collisions =
       (for { (_, coll) <- table.iterator_list if coll.length > 1 } yield coll).toList.flatten
     if (collisions.nonEmpty) {
       error(("Collision of file names due case-insensitivity:" :: collisions).mkString("\n  "))
     }
   }
 }
 
 
 final class Path private(private val elems: List[Path.Elem]) // reversed elements
 {
   def is_current: Boolean = elems.isEmpty
   def is_absolute: Boolean = elems.nonEmpty && elems.last.isInstanceOf[Path.Root]
   def is_root: Boolean = elems match { case List(Path.Root(_)) => true case _ => false }
   def is_basic: Boolean = elems match { case List(Path.Basic(_)) => true case _ => false }
 
   def +(other: Path): Path = new Path((other.elems :\ elems)(Path.apply_elem))
 
 
   /* implode */
 
   private def gen_implode(short: Boolean): String =
     elems match {
       case Nil => "."
       case List(Path.Root("")) => "/"
       case _ => elems.map(Path.implode_elem(_, short)).reverse.mkString("/")
     }
   def implode: String = gen_implode(false)
   def implode_short: String = gen_implode(true)
 
   override def toString: String = quote(implode)
 
 
   /* base element */
 
   private def split_path: (Path, String) =
     elems match {
       case Path.Basic(s) :: xs => (new Path(xs), s)
       case _ => error("Cannot split path into dir/base: " + toString)
     }
 
   def dir: Path = split_path._1
   def base: Path = new Path(List(Path.Basic(split_path._2)))
 
   def ext(e: String): Path =
     if (e == "") this
     else {
       val (prfx, s) = split_path
       prfx + Path.basic(s + "." + e)
     }
 
   def backup: Path =
   {
     val (prfx, s) = split_path
     prfx + Path.basic(s + "~")
   }
 
   def backup2: Path =
   {
     val (prfx, s) = split_path
     prfx + Path.basic(s + "~~")
   }
 
+  def platform_exe: Path =
+    if (Platform.is_windows) ext("exe") else this
+
   private val Ext = new Regex("(.*)\\.([^.]*)")
 
   def split_ext: (Path, String) =
   {
     val (prefix, base) = split_path
     base match {
       case Ext(b, e) => (prefix + Path.basic(b), e)
       case _ => (prefix + Path.basic(base), "")
     }
   }
 
   def drop_ext: Path = split_ext._1
   def get_ext: String = split_ext._2
 
 
   /* expand */
 
   def expand_env(env: Map[String, String]): Path =
   {
     def eval(elem: Path.Elem): List[Path.Elem] =
       elem match {
         case Path.Variable(s) =>
           val path = Path.explode(Isabelle_System.getenv_strict(s, env))
           if (path.elems.exists(_.isInstanceOf[Path.Variable]))
             error("Illegal path variable nesting: " + s + "=" + path.toString)
           else path.elems
         case x => List(x)
       }
 
     new Path(Path.norm_elems(elems.flatMap(eval)))
   }
 
   def expand: Path = expand_env(Isabelle_System.settings())
 
   def file_name: String = expand.base.implode
 
 
   /* source position */
 
   def position: Position.T = Position.File(implode)
 
 
   /* platform files */
 
   def file: JFile = File.platform_file(this)
   def is_file: Boolean = file.isFile
   def is_dir: Boolean = file.isDirectory
 
   def absolute_file: JFile = File.absolute(file)
   def canonical_file: JFile = File.canonical(file)
 
   def absolute: Path = File.path(absolute_file)
   def canonical: Path = File.path(canonical_file)
 }