src/Pure/PIDE/line.scala

   /* position */
 
   object Position
   {
     val zero: Position = Position()
+    val offside: Position = Position(line = -1)
 
     object Ordering extends scala.math.Ordering[Position]
     {
       def compare(p1: Position, p2: Position): Int = p1 compare p2
     }

       else start.print + ".." + stop.print
   }
 
 
   /* positions within document node */
+
+  object Node_Position
+  {
+    def offside(name: String): Node_Position = Node_Position(name, Position.offside)
+  }
 
   sealed case class Node_Position(name: String, pos: Position = Position.zero)
   {
     def line: Int = pos.line
     def column: Int = pos.column