Chagall Reference   Mirror block           12.5.93
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   This function can modify a block or the entire
   picture, providing a mirror image of it.

   Actually, to mirror a block only one piece of 
   information is needed, namely the axis at which
   reflection should take place:

   In this dialog you can choose between 'Horizontal'
   (X-axis) and 'Vertical' (Y-axis), whereby the
   axes run down the middle of the picture. 
   Alternatively in Chagall you can also mirror
   at any desired, freely definable axis. To do
   this activate 'To line' and click on 'Define'.
   You can now draw a line directly which Chagall
   will use as a reflection axis. The display will
   show you the line angle; deliberately, this 
   cannot be input numerically.

   As for most block functions you can finally set
   the Effective rangeFUNCTION for the function.
