src/Pure/PIDE/command.scala

 
   val proper_range: Text.Range =
     Text.Range(0,
       (length /: span.content.reverse.iterator.takeWhile(_.is_improper))(_ - _.source.length))
 
-  def source(range: Text.Range): String = source.substring(range.start, range.stop)
+  def source(range: Text.Range): String = range.substring(source)
 
 
   /* accumulated results */
 
   val init_state: Command.State =