isabelle_path: slightly more liberal root pattern;

--- a/src/Pure/System/isabelle_system.scala	Sat Jun 27 18:46:08 2009 +0200
+++ b/src/Pure/System/isabelle_system.scala	Sat Jun 27 19:24:34 2009 +0200
@@ -210,7 +210,7 @@
   /* isabelle_path */
 
   private val Platform_Root = new Regex("(?i)" +
-    Pattern.quote(platform_root) + """(?:\\|\z)(.*)""")
+    Pattern.quote(platform_root) + """(?:\\+|\z)(.*)""")
   private val Drive = new Regex("""([a-zA-Z]):\\*(.*)""")
 
   def isabelle_path(platform_path: String): String =