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 invcolor()          Swap fore/background values in a color attribute
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 Declaration
   color.hdr

 Syntax
   func uint invcolor extern
   param value uint uColorAttribute

 Arguments
   uColorAttribute is the color attribute to swap.

 Return
   A color attribute value.

 Description
   This function swaps the foreground and background values in a color
   attribute, i. e. bits 4...6 and bits 2...0 will be swapped. The function
   is handy to determine the corresponding inverse equivalent for a given
   color attribute.

 Example
   #define EXAMPLE_SCREEN
   #include example.hdr

   proc Test_invcolor
   __syscolor[ CLR_STD ] := BLUE_WHITE
   ? "Hello"                                   // displays white on blue
   __syscolor[ CLR_STD ] := invcolor( __syscolor[ CLR_STD ] )
   ? "Hello"                                   // displays blue on white
   __syscolor[ CLR_STD ] := invcolor( __syscolor[ CLR_STD ] )
   ? "Hello"                                   // displays white on blue again
   endproc

   proc main
   Test_invcolor()
   endproc