src/Pure/System/session.scala

         def revert(offset: Int): Int = (offset /: reverse_edits)((i, edit) => edit.revert(i))
         def lookup_command(id: Document.Command_ID): Option[Command] =
           state_snapshot.lookup_command(id)
         def state(command: Command): Command.State =
           try { state_snapshot.command_state(document, command) }
-          catch { case _: State.Fail => command.empty_state }
+          catch { case _: Document.State.Fail => command.empty_state }
       }
     }
 
     def act
     {