src/Pure/PIDE/document.scala

     val id: Document_ID.Version = Document_ID.none,
     val syntax: Outer_Syntax = Outer_Syntax.empty,
     val nodes: Nodes = Nodes.empty)
   {
     def is_init: Boolean = id == Document_ID.none
+
+    override def toString: String = "Version(" + id + ")"
   }
 
 
   /* changes of plain text, eventually resulting in document edits */