Chagall Reference   Invert mask        12.5.93
==============================================

   This functions permits one to flip the effect
   of the mask so that one can work exclusively 
   on those parts of the picture previously 
   protected by the mask. The soft transitions
   at the mask edges are retained.

   In some cases when the area one wishes to work
   on contains only a small range of colours (say 
   a background) it is far easier to generate a
   mask from this and then invert it to protect one
   or more foreground objects than to mask the object(s)
   directly.

