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 CHARWIN()
 Exchanges particular characters in a screen area.
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 Syntax

     CHARWIN([<nTopline>,<nLeftcolumn>,<nBottomline>,
        <nRightcolumn>, [<cNewcharacter|nNewcharacter>],
        [<cOldcharacter|nOldcharacter>]]) --> cNull

 Arguments

     <nTopline>  Designates the line for the top-left corner of the area.

     <nLeftcolumn>  Designates the column for the top-left corner the
     area.

     <nBottomline>  Designates the line for the bottom-right corner of
     the area.

     <nRightcolumn>  Designates  the column for the bottom-right corner
     of the area.

     <cNewcharacter|nNewcharacter>  Designates the new character for the
     screen area.  Specify the parameter as a numeric in the range of 0 to
     255 or as a character string.  The default value is the CLEARB.

     <cOldcharacter|nOldcharacter>  Designates which character to
     exchange.  Specify the parameter as a numeric in the range of  0 to 255
     or as a character string.  The default is to exchange all characters.

     ()  When no parameter is designated, the function replaces all
     characters in the screen area with CLEARB.

 Returns

     The function always returns a null string.

 Description

     Within a screen area, CHARWIN() replaces all characters or just a
     particular character with a new one.  If <cNewcharacter|nNewcharacter>
     is not designated, the function uses the character set with SETCLEARB(),
     where the preset value is CHR(255).  The upper-left and lower-right
     corner rows and columns are given for the borders of the area.  If these
     arguments are missing, then the entire screen area is exchanged.

 Examples

     .  Exchange all characters for CLEARB:

        SETCLEARB("X")       // Exchanges within CHARWIN() entire screen

     .  Exchange all "A" for "B":

        CHARWIN(10, 10, 20, 70, "B", "A")    // Always returns a ("") null
                                             // string

     .  Numeric characters can also be designated:

        CHARWIN(10, 10, 20, 70, 66, 65)      // A = 65, B = 66