Tools for simple diagrams (arrows and text strings).

   yardstick.R -> diagram.R
   yardstick.Rd
   plot.yardstick.Rd
   plot.onearrow.Rd  [New file]
   plot.textstring.Rd [New file]
      Rebuilt class 'yardstick' and added new classes
      'onearrow' and 'textstring', for building diagrams.

   DESCRIPTION
   NEWS
      updated.