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 CLWIN()
 Clears character and attribute from a screen area
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 Syntax

     CLWIN([<nTopLine>], [<nLeftCol>], [<nBottomLine>],
        [<nRightColumn>]) --> cNull

 Arguments

     <nTopLine>  Designates the topmost line where clearing should begin.
     The default is the cursor line.

     <nLeftCol>  Designates the leftmost column where erasure should
     begin.  The default is the cursor column.

     <nBottomLine>  Designates the bottommost line to clear.  The default
     is the last screen line or window line.

     <nRightColumn>  Designates the rightmost column to clear.  The
     default is the right screen border or window border.

 Returns

     CLWIN() always returns a null string.

 Description

     Implement CLWIN() when you need to clear to black without the use of the
     standard attribute and character.  This function is helpful when you use
     a monochrome video card.  A significant advantage of CLWIN() over
     CLEARWIN(), is the omission of the <cAttr|nAtt> and
     <cCharacter|nCharacter> parameters, because it increases the speed of
     CLWIN().  The function is faster because it does not note the CLEARA and
     CLEARB settings.

     CLWIN() has some advantages over the Clipper @ Row, Column commands:

     1. This function takes the returned value of the character string type
        and combines it with string outputs  (e.g., LIST...CLWIN()).

     2. The cursor position is not affected.

     3. The parameters are optional.  If they are not specified, the function
        clears from the current cursor position.

 Note

     .  If no parameters are specified for the line or column, the
        function automatically orients itself to the current cursor position.

 Examples

     .  Clear from the cursor position to the end of the screen area:

        CLWIN()

     .  Do the same from the current cursor position in line 23:

        CLWIN(23)

     .  Clear from line 23, column 15, to the end of line 24:

        CLWIN(23, 15, 24)

     .  Clear a window between line 23, column 15, and line 24, column
        70:

        CLWIN(23, 15, 24, 70)